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.

[codecov[bot]]

Codecov Report

Merging #22 into master will not change coverage. The diff coverage is n/a.

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  Coverage   90.67%   90.67%           
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  Hits          243      243           
  Misses         21       21           
  Partials        4        4

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[thedrow]

Thanks for the contribution!