Don't write bytecode when invoking Python tests

[cottsay]

This should help to avoid writing compiled bytecode into the source directories of Python packages during test invocation.

https://docs.python.org/3/using/cmdline.html#envvar-PYTHONDONTWRITEBYTECODE

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (291f16c) 83.30% compared to head (6cb4447) 82.78%.

Files	Patch %	Lines
colcon_core/task/python/test/__init__.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
- Coverage   83.30%   82.78%   -0.52%     
==========================================
  Files          65       65              
  Lines        3762     3765       +3     
  Branches      727      649      -78     
==========================================
- Hits         3134     3117      -17     
- Misses        552      568      +16     
- Partials       76       80       +4     

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