Add compatibility with pip==19.3 for removed Link.is_artifact property

[atugushev]

Remove Link.is_artifact usage in favour of is_vcs_url.

See pypa/pip#7059

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 #904 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #904   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files          35       35           
  Lines        2247     2247           
  Branches      288      288           
=======================================
  Hits         2226     2226           
  Misses         13       13           
  Partials        8        8

Impacted Files	Coverage Δ
piptools/repositories/pypi.py	95.12% <100%> (ø)	⬆️

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[codingjoe]

Should be ok, but I have one concern about using the method over the is_vcs property. I am not so fluent in the pip changes though.

[codingjoe]

👍

[atugushev]

Thanks @codingjoe!