Document Python 2 deprecation (#6910)

Author: crwilcox

packages/google-cloud-core/README.rst

@@ -29,3 +29,11 @@ to `Python Development Environment Setup Guide`_ for Google Cloud Platform.
 
 .. _Python Development Environment Setup Guide: https://cloud.google.com/python/setup
 
+
+Supported Python Versions
+-------------------------
+Python >= 3.4
+
+Deprecated Python Versions
+--------------------------
+Python == 2.7. Python 2.7 support will be removed on January 1, 2020.

packages/google-cloud-core/setup.py

@@ -72,6 +72,7 @@
         "Programming Language :: Python :: 3.4",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
+        'Programming Language :: Python :: 3.7',
         "Operating System :: OS Independent",
         "Topic :: Internet",
     ],
@@ -80,6 +81,7 @@
     namespace_packages=namespaces,
     install_requires=dependencies,
     extras_require=extras,
+    python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*',
     include_package_data=True,
     zip_safe=False,
 )