mypy error does not stop GitHub Actions

[tushuhei]

As shown in https://github.com/google/budoux/runs/6555048469, our "Style Check" GitHub Action won't be interrupted even if mypy finds an error. Any mypy error should pause the process to flag contributors about its type error.

Run sasanquaneuf/mypy-github-action@a0c442aa252655d7736ce6696e06227ccdd62870
/opt/hostedtoolcache/Python/3.9.12/x64/bin/mypy .
build/lib/budoux/__init__.py: error: Duplicate module named "budoux" (also at
"./budoux/__init__.py")
build/lib/budoux/__init__.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
build/lib/budoux/__init__.py: note: Common resolutions include: a) using `--exclude` to avoid checking one of them, b) adding `__init__.py` somewhere, c) using `--explicit-package-bases` or adjusting MYPYPATH
Found 1 error in 1 file (errors prevented further checking)

[tushuhei]

Fixed by #70