Respect --pre option in requirements.in file

[atugushev]

Fixes #819.

Changelog-friendly one-liner: Respect --pre option in a requirements.in file

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

@@            Coverage Diff            @@
##           master    #822      +/-   ##
=========================================
+ Coverage    98.8%   98.8%   +<.01%     
=========================================
  Files          36      36              
  Lines        2182    2184       +2     
  Branches      279     280       +1     
=========================================
+ Hits         2156    2158       +2     
  Misses         15      15              
  Partials       11      11

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

---

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[codecov]

Codecov Report

Merging #822 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #822      +/-   ##
=========================================
+ Coverage    98.8%   98.8%   +<.01%     
=========================================
  Files          36      36              
  Lines        2182    2184       +2     
  Branches      279     280       +1     
=========================================
+ Hits         2156    2158       +2     
  Misses         15      15              
  Partials       11      11

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

---

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 0eeb7de...70c8fc4. Read the comment docs.

[atugushev]

pip-tools v3.8.0 is released