Deprecate Python 3.6 support.

[mwichmann]

No claim is made about when it will actually be dropped, but some notice seems polite. Reworded the text of the version warning.

RELEASE contains a note on how to shut up the warning if people don't want to see it.

Contributor Checklist:

• [X] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt (and read the README.rst)
• [X] I have updated the appropriate documentation