chore(deps)!: require 3.6+ (#961)

Removes 2.7 and 3.5 from the test suite and explicitly requires >= 3.6. Also adds tests for 3.9

Fixes #704 🦕
diff --git a/setup.py b/setup.py
index bf0a8f4..189f5fd 100644
--- a/setup.py
+++ b/setup.py
@@ -21,11 +21,8 @@
 
 import sys
 
-if sys.version_info < (2, 7):
-    print("google-api-python-client requires python version >= 2.7.", file=sys.stderr)
-    sys.exit(1)
-if (3, 1) <= sys.version_info < (3, 4):
-    print("google-api-python-client requires python3 version >= 3.4.", file=sys.stderr)
+if sys.version_info < (3, 6):
+    print("google-api-python-client requires python3 version >= 3.6.", file=sys.stderr)
     sys.exit(1)
 
 import io
@@ -61,18 +58,17 @@
     author_email="googleapis-packages@google.com",
     url="https://github.com/googleapis/google-api-python-client/",
     install_requires=install_requires,
-    python_requires=">=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*",
+    python_requires=">=3.6",
     packages=packages,
     package_data={},
     license="Apache 2.0",
     keywords="google api client",
     classifiers=[
-        "Programming Language :: Python :: 2",
-        "Programming Language :: Python :: 2.7",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",