Prevent to uninstall stdlib and dev packages

[atugushev]

Fixes #690.

Changelog-friendly one-liner: Prevent to uninstall stdlib and dev packages

Contributor checklist

• Provided the tests for the changes
• Requested (or received) a review from another contributor
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md afterwards).

[codecov]

Codecov Report

Merging #718 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #718      +/-   ##
==========================================
+ Coverage   85.17%   85.18%   +<.01%     
==========================================
  Files          31       31              
  Lines        2064     2065       +1     
  Branches      309      309              
==========================================
+ Hits         1758     1759       +1     
  Misses        235      235              
  Partials       71       71

Impacted Files	Coverage Δ
tests/test_sync.py	98.21% <ø> (ø)	⬆️
piptools/sync.py	78.94% <100%> (+0.22%)	⬆️

---

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 62a5b4d...7c46006. Read the comment docs.

[atugushev]

@blueyed @suutari-ai
Guys, i've squashed commits into one. PR is ready to merge, if you approve it.

[atugushev]

Thank you @vphilippon!