Enforce minimum Python version

[johnthagen]

This helps pip do version resolution based on the current version of Python running when django-downloadview is requested to be installed.

• https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#python-requires

[codecov[bot]]

Codecov Report

Merging #199 (dd2e148) into master (64e3682) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   85.52%   85.52%           
=======================================
  Files          72       72           
  Lines        1313     1313           
=======================================
  Hits         1123     1123           
  Misses        190      190           

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)

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