Bring coverage back to 100%

[pquentin]

The real reason for this pull request is that we forgot to mark our Rlock as not covered, but I also used this opportunity to switch to a different .coveragerc string.

Indeed, it's true that threading support is "platform-specific" since it's optional in Python 3.6, but since it's always available in Python 3.7 (https://docs.python.org/3/library/threading.html), using "Python 3.6" will remind us to remove the shim when we drop support for Python 3.6 next year.

[codecov]

Codecov Report

Merging #2174 (676f79e) into main (6d799ce) will increase coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #2174      +/-   ##
===========================================
+ Coverage   99.86%   100.00%   +0.13%     
===========================================
  Files          26        25       -1     
  Lines        2256      2253       -3     
===========================================
  Hits         2253      2253              
+ Misses          3         0       -3     

Impacted Files	Coverage Δ
src/urllib3/_collections.py	100.00% <ø> (ø)

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[sethmlarson]

LGTM! Didn't know Python 3.7 made the module non-optional, TIL :)