Fix pip>=19.1 compatibility

[atugushev]

Closes issue #794.

Changelog-friendly one-liner: Fix pip>=19.1 compatibility

Contributor checklist

• Provided the tests for the changes.
• Requested a review from another contributor.
• 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).

[codecov]

Codecov Report

Merging #795 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #795      +/-   ##
==========================================
+ Coverage   98.06%   98.06%   +<.01%     
==========================================
  Files          34       34              
  Lines        1962     1965       +3     
  Branches      256      257       +1     
==========================================
+ Hits         1924     1927       +3     
  Misses         26       26              
  Partials       12       12

Impacted Files	Coverage Δ
piptools/repositories/pypi.py	89.34% <100%> (+0.19%)	⬆️

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[vphilippon]

LGTM 👍
Thanks for the quick PR!

I'll get on the release right away.

[naggie]

Thanks for fixing this so fast, @atugushev