Fix support for pip versions pre-21.3

[MrMino]

Fixes #73

Tested this on (I've ran pip-check-reqs and pip-missing-reqs watching if there are any ImportErrors) all versions of pip above 10.0.

All work except pip==20.0 which had its own problems and IIRC was yanked from PyPI.

[codecov-commenter]

Codecov Report

Merging #74 (f649aea) into master (f0716ac) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #74   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          587       590    +3     
=========================================
+ Hits           587       590    +3     

Impacted Files	Coverage Δ
pip_check_reqs/__init__.py	100.00% <100.00%> (ø)
pip_check_reqs/common.py	100.00% <100.00%> (ø)

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[adamtheturtle]

Thank you @MrMino - if you're interested please email me / Twitter DM me about being added to this project as a maintainer.

[MrMino]

@adamtheturtle sure, I'd love that, I'll do so as soon as I'm next to my PC :).