Fix sync with a temp file on Windows

[atugushev]

Fixes #722

Changelog-friendly one-liner: Fix sync with a temporary requirement file on Windows

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 #723 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
+ Coverage   85.18%   85.26%   +0.07%     
==========================================
  Files          31       31              
  Lines        2065     2076      +11     
  Branches      309      309              
==========================================
+ Hits         1759     1770      +11     
  Misses        235      235              
  Partials       71       71

Impacted Files	Coverage Δ
tests/test_sync.py	98.34% <100%> (+0.13%)	⬆️
piptools/sync.py	79.38% <100%> (+0.43%)	⬆️

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