keg: delete_pyc_files! should also remove __pycache__

[lembacon]

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As Python 3 becomes the default Python in the core tap, bottles built against Python 3 become a thing. Both .pycand .pyo have already been removed from bottles, we should also remove __pycache__. Otherwise people who have PYTHONDONTWRITEBYTECODE set will find this really annoying.

[MikeMcQuaid]

Thanks again @lembacon!

[MikeMcQuaid]

Oh, this was your first Homebrew/brew PR! In that case:
Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @lembacon 😁

[MikeMcQuaid]

FYI I reverted my two tweak commits because __pycache__ is a directory containing those cache files so can only be removed after they are.