Normalize cache_dir path to be compatible with pip v20

[kammala]

Fixes #1061

Always use normalized path for cache directory as it is required in newer versions of pip

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 #1062 into master will decrease coverage by 0.96%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1062      +/-   ##
==========================================
- Coverage   99.26%   98.29%   -0.97%     
==========================================
  Files          34       34              
  Lines        2446     2463      +17     
  Branches      306      307       +1     
==========================================
- Hits         2428     2421       -7     
- Misses         10       30      +20     
- Partials        8       12       +4     

Impacted Files	Coverage Δ
tests/test_repository_pypi.py	81.11% <0.00%> (-18.89%)	⬇️
piptools/repositories/pypi.py	90.33% <0.00%> (-2.35%)	⬇️
piptools/utils.py	98.67% <0.00%> (-1.33%)	⬇️

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[pradyunsg]

While I'm here -- heads up that we'd be making changes to pip's resolver interface in the coming release; I suggest you track how master evolves (and provide inputs!) to make sure y'all aren't completely blindsided. :)

[atugushev]

@pradyunsg thanks for the heads-up! Eagerly waiting for new resolver 😊

[atugushev]

LGTM 👍

[atugushev]

@kammala thanks for your first contribution! 👍 👏

[atugushev]

pip-tools v4.5.0 is released.