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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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