Modify shebang on python scripts

Change shebangs to `#!/usr/bin/env python` instead of
`#!/usr/bin/env python2.7`.
diff --git a/tools/run_tests/artifacts/artifact_targets.py b/tools/run_tests/artifacts/artifact_targets.py
index e0658f4..04702ba 100644
--- a/tools/run_tests/artifacts/artifact_targets.py
+++ b/tools/run_tests/artifacts/artifact_targets.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/artifacts/distribtest_targets.py b/tools/run_tests/artifacts/distribtest_targets.py
index a7535b3..90bbde8 100644
--- a/tools/run_tests/artifacts/distribtest_targets.py
+++ b/tools/run_tests/artifacts/distribtest_targets.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/artifacts/package_targets.py b/tools/run_tests/artifacts/package_targets.py
index d490f57..2547f20 100644
--- a/tools/run_tests/artifacts/package_targets.py
+++ b/tools/run_tests/artifacts/package_targets.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/performance/bq_upload_result.py b/tools/run_tests/performance/bq_upload_result.py
index e2e1393..3bac119 100755
--- a/tools/run_tests/performance/bq_upload_result.py
+++ b/tools/run_tests/performance/bq_upload_result.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/python_utils/antagonist.py b/tools/run_tests/python_utils/antagonist.py
index 857addf..111839c 100755
--- a/tools/run_tests/python_utils/antagonist.py
+++ b/tools/run_tests/python_utils/antagonist.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/python_utils/filter_pull_request_tests.py b/tools/run_tests/python_utils/filter_pull_request_tests.py
index 041e349..e013376 100644
--- a/tools/run_tests/python_utils/filter_pull_request_tests.py
+++ b/tools/run_tests/python_utils/filter_pull_request_tests.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/python_utils/port_server.py b/tools/run_tests/python_utils/port_server.py
index e9b3f7f..dbd32ef 100755
--- a/tools/run_tests/python_utils/port_server.py
+++ b/tools/run_tests/python_utils/port_server.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/run_build_statistics.py b/tools/run_tests/run_build_statistics.py
index e33e01e..dd11a45 100755
--- a/tools/run_tests/run_build_statistics.py
+++ b/tools/run_tests/run_build_statistics.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/run_interop_tests.py b/tools/run_tests/run_interop_tests.py
index ace142e..29d5a9a 100755
--- a/tools/run_tests/run_interop_tests.py
+++ b/tools/run_tests/run_interop_tests.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/run_microbenchmark.py b/tools/run_tests/run_microbenchmark.py
index 9471110..57b2636 100755
--- a/tools/run_tests/run_microbenchmark.py
+++ b/tools/run_tests/run_microbenchmark.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2017, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/run_performance_tests.py b/tools/run_tests/run_performance_tests.py
index c79f47a..35d20be 100755
--- a/tools/run_tests/run_performance_tests.py
+++ b/tools/run_tests/run_performance_tests.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/run_stress_tests.py b/tools/run_tests/run_stress_tests.py
index 3a2661a..4eea021 100755
--- a/tools/run_tests/run_stress_tests.py
+++ b/tools/run_tests/run_stress_tests.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/run_tests_matrix.py b/tools/run_tests/run_tests_matrix.py
index 191228e..b9a9bd5 100755
--- a/tools/run_tests/run_tests_matrix.py
+++ b/tools/run_tests/run_tests_matrix.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/sanity/check_sources_and_headers.py b/tools/run_tests/sanity/check_sources_and_headers.py
index 0bb92fd..f2e0bfe 100755
--- a/tools/run_tests/sanity/check_sources_and_headers.py
+++ b/tools/run_tests/sanity/check_sources_and_headers.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/sanity/check_test_filtering.py b/tools/run_tests/sanity/check_test_filtering.py
index aceee6d..ba03f11 100755
--- a/tools/run_tests/sanity/check_test_filtering.py
+++ b/tools/run_tests/sanity/check_test_filtering.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 
 # Copyright 2016, Google Inc.
 # All rights reserved.
diff --git a/tools/run_tests/sanity/check_version.py b/tools/run_tests/sanity/check_version.py
index d8162fb..d247260 100755
--- a/tools/run_tests/sanity/check_version.py
+++ b/tools/run_tests/sanity/check_version.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 
 # Copyright 2016, Google Inc.
 # All rights reserved.
diff --git a/tools/run_tests/sanity/core_banned_functions.py b/tools/run_tests/sanity/core_banned_functions.py
index c1069cc..c3c3cbe 100755
--- a/tools/run_tests/sanity/core_banned_functions.py
+++ b/tools/run_tests/sanity/core_banned_functions.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 
 # Copyright 2016, Google Inc.
 # All rights reserved.
diff --git a/tools/run_tests/stress_test/print_summary.py b/tools/run_tests/stress_test/print_summary.py
index cb1a339..6f4ada2 100755
--- a/tools/run_tests/stress_test/print_summary.py
+++ b/tools/run_tests/stress_test/print_summary.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/stress_test/run_on_gke.py b/tools/run_tests/stress_test/run_on_gke.py
index a2a5f42..b190ebd 100755
--- a/tools/run_tests/stress_test/run_on_gke.py
+++ b/tools/run_tests/stress_test/run_on_gke.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015-2016, Google Inc.
 # All rights reserved.
 #
diff --git a/tools/run_tests/task_runner.py b/tools/run_tests/task_runner.py
index fdc4668..0ec7efb 100755
--- a/tools/run_tests/task_runner.py
+++ b/tools/run_tests/task_runner.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2016, Google Inc.
 # All rights reserved.
 #