Add .gitignore to deal with __pycache__ artifacts

[cottsay]

I don't know why I've put up with this for so long, but this has already been done on several other colcon repositories.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (7b70e61) 81.88% compared to head (6e45e48) 81.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #563 +/- ## ======================================= Coverage 81.88% 81.88% ======================================= Files 62 62 Lines 3648 3648 Branches 705 705 ======================================= Hits 2987 2987 Misses 609 609 Partials 52 52 ```

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