Declare pep517 dependency

[atugushev]

Fixes #1352

Changelog-friendly one-liner: Declare pep517 dependency

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).

[codecov]

Codecov Report

Merging #1353 (c47fb58) into master (3fa72d0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1353      +/-   ##
==========================================
- Coverage   99.65%   99.65%   -0.01%     
==========================================
  Files          33       33              
  Lines        2915     2914       -1     
  Branches      308      308              
==========================================
- Hits         2905     2904       -1     
  Misses          5        5              
  Partials        5        5              

Impacted Files	Coverage Δ
piptools/scripts/compile.py	100.00% <100.00%> (ø)
piptools/repositories/pypi.py	97.10% <0.00%> (-0.02%)	⬇️

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[webknjaz]

🤔

[vphilippon]

Yup, that makes sense, pep517 would declare the (transitive) dependency on importlib_metadata for Python < 3.8.
And a bit less impact if pip moves this vendored part (not that we are resilient to pip internal changes at all, but that's one thing less :) )
👍

[atugushev]

Thanks @webknjaz, @vphilippon and @astrojuanlu 🙏🏼