Use contextlib in test_profile_parser

[s-t-e-v-e-n-k]

Since the project no longer declares a requirement of contextlib2, and
the lowest version of Python supported is 3.7, we should remove the use
of contextlib2. Switch the import to contextlib.

[codecov]

Codecov Report

Merging #590 (a18a012) into master (dcbc6e0) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head a18a012 differs from pull request most recent head 4ca96d9. Consider uploading reports for the commit 4ca96d9 to get more accurate results

@@           Coverage Diff           @@
##           master     #590   +/-   ##
=======================================
  Coverage   84.59%   84.60%           
=======================================
  Files          52       52           
  Lines        2071     2072    +1     
=======================================
+ Hits         1752     1753    +1     
  Misses        319      319           

Impacted Files	Coverage Δ
silk/model_factory.py	82.96% <0.00%> (-0.08%)	⬇️
silk/collector.py	89.78% <0.00%> (+0.07%)	⬆️
silk/middleware.py	89.42% <0.00%> (+0.10%)	⬆️

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[albertyw]

Can you also edit requirements.txt and remove the contextlib2 dependency there too?