Fix pip>=19.2 compatibility

[atugushev]

Closes #856.

TODO

• fix compatibility
• refactore piptools/pip.py and tests with get_pip_command
• wait for release PR Update Link.is_hash_allowed() to accept hashes=None pypa/pip#6774, since it breaks pip-tools

Changelog-friendly one-liner: Fix pip>=19.2 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).

Refs #853.

[codecov]

Codecov Report

Merging #857 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #857     +/-   ##
=========================================
+ Coverage   98.85%   99.06%   +0.2%     
=========================================
  Files          36       35      -1     
  Lines        2191     2147     -44     
  Branches      285      284      -1     
=========================================
- Hits         2166     2127     -39     
+ Misses         15       12      -3     
+ Partials       10        8      -2

Impacted Files	Coverage Δ
piptools/repositories/local.py	91.11% <100%> (+0.41%)	⬆️
piptools/utils.py	100% <100%> (ø)	⬆️
tests/test_repositories.py	100% <100%> (ø)	⬆️
tests/test_top_level_editable.py	100% <100%> (ø)	⬆️
piptools/scripts/compile.py	100% <100%> (ø)	⬆️
tests/test_repository_pypi.py	100% <100%> (ø)	⬆️
tests/test_cli_compile.py	100% <100%> (ø)	⬆️
piptools/repositories/pypi.py	95.23% <100%> (+2.6%)	⬆️
tests/test_repository_local.py	100% <100%> (ø)	⬆️
tests/conftest.py	97.36% <100%> (+0.1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6a116c7...072e72b. Read the comment docs.

[atugushev]

Finally it's all green!

@vphilippon @blueyed kindly ping you guys, review it please if you have a time.

[blueyed]

Nice!

[blueyed]

I've not looked too closely, but given that CI will like it I think it's good!
Thanks!

[atugushev]

Thanks @auvipy @blueyed !

[atugushev]

pip-tools v4.0.0 is released.