Add "current python" factor

[adamchainz]

Fixes #14.

[codecov]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           82        87    +5     
  Branches        11        12    +1     
=========================================
+ Hits            82        87    +5     

Impacted Files	Coverage Δ
src/tox_factor/hooks.py	100.00% <ø> (ø)
src/tox_factor/factor.py	100.00% <100.00%> (ø)
src/tox_factor/test.py	100.00% <100.00%> (ø)

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[adamchainz]

Some of the isort changes here are due to the release of isort 5, and will be superceded by #16.