[WIP] Parse extras_require when compile setup.py

[atugushev]

Closes #625.

Changelog-friendly one-liner: Parse extras_require when compile setup.py file.

TODO

• handle extras_require={'dev': "small-fake-a==0.1"}
• research how pip handles extras_require via pip install -e .[dev]

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 #877 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #877      +/-   ##
==========================================
+ Coverage   99.05%   99.06%   +0.01%     
==========================================
  Files          35       35              
  Lines        2231     2256      +25     
  Branches      285      290       +5     
==========================================
+ Hits         2210     2235      +25     
  Misses         13       13              
  Partials        8        8

Impacted Files	Coverage Δ
tests/test_cli_compile.py	100% <100%> (ø)	⬆️
piptools/scripts/compile.py	100% <100%> (ø)	⬆️

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[atugushev]

I'll close it in favor of #908.