Add python_requires arg to setup.cfg (#273)

* Add python_requires arg to setup.py.

This prevents pip from attempting to install new python-jose packages on
unsupported Python versions.

See https://packaging.python.org/tutorials/packaging-projects/#configuring-metadata

* Bumb python_requires to 3.7.

* python 3.9

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Co-authored-by: Asher Foa <asher@userclouds.com>