Merge pull request #15658 from jtattermusch/python_interop_build_27

Only build interop tests with python2.7
diff --git a/tools/dockerfile/interoptest/grpc_interop_python/build_interop.sh b/tools/dockerfile/interoptest/grpc_interop_python/build_interop.sh
index a8898f8..7917e1c 100755
--- a/tools/dockerfile/interoptest/grpc_interop_python/build_interop.sh
+++ b/tools/dockerfile/interoptest/grpc_interop_python/build_interop.sh
@@ -28,4 +28,5 @@
 
 cd /var/local/git/grpc
 
-tools/run_tests/run_tests.py -l python -c opt --build_only
+# interop tests only run using python2.7 currently (and python build is slow)
+tools/run_tests/run_tests.py -l python --compiler python2.7 -c opt --build_only