Fix url_to_path() import

[atugushev]

Function url_to_path() has been moved to pip._internal.utils.urls.
See pypa/pip#7089

Changelog-friendly one-liner: Add compatibility with pip==19.3.

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 #915 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #915      +/-   ##
==========================================
+ Coverage   99.06%   99.08%   +0.01%     
==========================================
  Files          35       35              
  Lines        2247     2285      +38     
  Branches      288      301      +13     
==========================================
+ Hits         2226     2264      +38     
  Misses         13       13              
  Partials        8        8

Impacted Files	Coverage Δ
tests/test_cli_compile.py	100% <0%> (ø)	⬆️
piptools/writer.py	99.31% <0%> (+0.17%)	⬆️

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[codingjoe]

👍

[atugushev]

Thanks as always @codingjoe!