🔖 Allow pip 21.1

[ddelange]

Assuming pip 21.1.0 will be the first release to include pypa/pip#9405

[codecov-io]

Codecov Report

Merging #49 (9f7c8eb) into master (96390bd) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
=======================================
  Coverage   77.39%   77.39%           
=======================================
  Files          29       29           
  Lines        2729     2729           
  Branches      645      645           
=======================================
  Hits         2112     2112           
  Misses        444      444           
  Partials      173      173           

Impacted Files	Coverage Δ
src/pipgrip/pipper.py	75.00% <0.00%> (ø)

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