[python27] Deprecate support for python27

[mmcky]

This should be done in January 2020

[mmcky]

@nthiery do you still require python27 support for sage?

[nthiery]

Sage is on a good migration path to Python 3; presumably the first official release that will be Python 3 by default will come early next winter. In addition, in my use case I actually use a separate Python interpreter.

So all in all, deprecating Python starting from January 2020 is fair enough. If it can keep working for a couple extra months, that's a bonus.

Thanks for asking!

[mmcky]

thanks @nthiery - no problem - I will leave python27 checking (and code) in place until March/April 2020 -- but will start NotImplementedError for any incoming development work.