Revert back piptools._compat.contextlib

[atugushev]

The module pip._vendor.contextlib2 has been recently removed in pypa/pip#9384, which breaks pip-tools (see failing cron jobs for the pip's master branch).

This PR brings back the piptools._compat.contextlib.nullcontext() conext manager removed in #1283 .

/cc @jdufresne

[codecov]

Codecov Report

Merging #1338 (f821c17) into master (9aeeed1) will decrease coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1338      +/-   ##
==========================================
- Coverage   99.62%   99.21%   -0.42%     
==========================================
  Files          33       33              
  Lines        2929     2927       -2     
  Branches      319      319              
==========================================
- Hits         2918     2904      -14     
- Misses          5       11       +6     
- Partials        6       12       +6     

Impacted Files	Coverage Δ
piptools/repositories/pypi.py	92.51% <100.00%> (-4.75%)	⬇️

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[atugushev]

@jdufresne thanks!