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

---------

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

Author: avian2

setup.cfg

@@ -32,6 +32,7 @@ classifiers =
 
 [options]
 packages = find:
+python_requires = >=3.9
 install_requires =
     ecdsa != 0.15
     rsa >=4.0, <5.0, !=4.4, !=4.1.1