Add `python_requires` to declare Python2.7 support drop.

[Vincent Philippon]

Hello!

Is it possible to populate this field in setup.py please: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

This will populate the package metadata so that pip stops attempting to install the more recent version on Python 2.7.

Although this will require to also yank the pyrsistent 0.17.0 release afterwards, or else pip will continue to keep trying to install this incompatible version on Python2.7 environments.
Ref: https://pypi.org/help/#yanked
That's done via the PyPI maintainer interface.

Thanks!

[Tobias Gustafsson]

v0.17.0 yanked and v0.17.1 with guard released. Thanks!