use pytest hook to redirect pytest warnings to stderr

[dirk-thomas]

To avoid using the same redirect in non-colcon pytest invocations the custom hook is only changing the behavior when the environment variable COLCON is set.

[codecov]

Codecov Report

Merging #338 into master will decrease coverage by 0.39%.
The diff coverage is 5.88%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
- Coverage   80.98%   80.59%   -0.40%     
==========================================
  Files          55       56       +1     
  Lines        3219     3236      +17     
  Branches      534      536       +2     
==========================================
+ Hits         2607     2608       +1     
- Misses        576      591      +15     
- Partials       36       37       +1     

Impacted Files	Coverage Δ
colcon_core/pytest/hooks.py	0.00% <0.00%> (ø)
colcon_core/command.py	86.52% <50.00%> (-0.39%)	⬇️

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[dirk-thomas]

Just rebased thinking I could get Codecov to turn green... I guess I was wrong 😞