Add pip>=20.1 support

[atugushev]

Changelog-friendly one-liner: Add pip>=20.1 support.

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

Note codecov patch check is failed because this PR is touched uncovered lines

[codecov]

Codecov Report

Merging #1088 into master will increase coverage by 0.00%.
The diff coverage is 84.00%.

@@           Coverage Diff           @@
##           master    #1088   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          34       34           
  Lines        2443     2453   +10     
  Branches      302      306    +4     
=======================================
+ Hits         2429     2439   +10     
  Misses          8        8           
  Partials        6        6           

Impacted Files	Coverage Δ
piptools/repositories/pypi.py	93.78% <69.23%> (+0.07%)	⬆️
piptools/repositories/local.py	96.22% <100.00%> (+0.22%)	⬆️
piptools/scripts/compile.py	100.00% <100.00%> (ø)
piptools/scripts/sync.py	100.00% <100.00%> (ø)
piptools/utils.py	100.00% <100.00%> (ø)
tests/test_repository_pypi.py	100.00% <100.00%> (ø)

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[atugushev]

@jazzband/pip-tools

pip==20.1 is on the way pypa/pip#7951 so this PR is ready to review!

[atugushev]

Thanks, @auvipy.