Pass python_requires argument to setuptools

[jdufresne]

Helps pip decide what version of the library to install.

https://packaging.python.org/tutorials/distributing-packages/#python-requires

If your project only runs on certain Python versions, setting the
python_requires argument to the appropriate PEP 440 version specifier
string will prevent pip from installing the project on other Python
versions.

https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords

python_requires

A string corresponding to a version specifier (as defined in PEP 440)
for the Python version, used to specify the Requires-Python defined in
PEP 345.