Add more messaging about 2.7 deprecation

[busunkim96]

Currently we mention 2.7 deprecation in the body of the README.rst files for APIs. There may be more actions we can take to signal to users that we will not be supporting Python 2 post Jan 1, 2020.

• Make deprecation warning more prominent in README and library reference documentation. docs: add python 2 EOL warning to reference documentation #9036
• Add deprecation warning to top-level repo README.
• Raise warnings when Python < 3.5 is detected at install time, run time, or both.

See https://python3statement.org/practicalities/ for more ideas.

[parthea]

I'm going to close this issue as we've dropped python 2 support in the default branch in most googleapis python repos but please feel free to re-open it.