Ignore .mypy_cache at all levels

Summary: This got generated at the top level for me.

Test Plan:

mkdir .mypy_cache
touch .mypy_cache/somefile
mkdir test/.mypy_cache
touch test/.mypy_cache/somefile
git diff  # verify both .mypy_cache directories are ignored

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9413