FIX: Pip 20 support for importing PipSession

[BaillyM]

FIX: Pip 20 support for importing PipSession

Tox wasn't correctly testing for me. Config slightly simplified and pip20+ added.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@58a1642). Click here to learn what that means.
The diff coverage is 14.28%.

@@            Coverage Diff            @@
##             master      #15   +/-   ##
=========================================
  Coverage          ?   89.39%           
=========================================
  Files             ?        1           
  Lines             ?       66           
  Branches          ?        0           
=========================================
  Hits              ?       59           
  Misses            ?        7           
  Partials          ?        0           

Impacted Files	Coverage Δ
pkgversion/pkgversion.py	89.39% <14.28%> (ø)

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