Issue #20 - refactor for Python 3.7 and Pipenv

[bear]

Update all code and tests to use Python 3.7

• switch to use Pipenv and pylint
• update Makefile and setup.cfg for the changes
• clean up a lot of linting issues

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@91a05fb). Click here to learn what that means. The diff coverage is 83.87%.

@@            Coverage Diff            @@
##             master      #21   +/-   ##
=========================================
  Coverage          ?   75.77%           
=========================================
  Files             ?        5           
  Lines             ?      289           
  Branches          ?        0           
=========================================
  Hits              ?      219           
  Misses            ?       70           
  Partials          ?        0

Impacted Files	Coverage Δ
ronkyuu/__init__.py	100% <ø> (ø)
ronkyuu/relme.py	81.81% <100%> (ø)
ronkyuu/validators.py	63.82% <100%> (ø)
ronkyuu/tools.py	75.36% <75%> (ø)
ronkyuu/webmention.py	76.37% <80.95%> (ø)

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