Pass python_requires argument to setuptools

[jdufresne]

Helps pip decide what version of the library to install. This is
especially useful when removing support for EOL Python versions.

https://packaging.python.org/guides/distributing-packages-using-setuptools/#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.

[coveralls]

Pull Request Test Coverage Report for Build 70

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-8.7%) to 68.011%

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Totals
Change from base Build 62:	-8.7%
Covered Lines:	489
Relevant Lines:	719

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💛 - Coveralls

[jpadilla]

This is neat, thanks!