Fix MyPy linting

[haplo]

Currently failing because mypy 0.9.0 unbundled third-party type stubs, which now need to be installed separately.

mypy .
pylsp_black/plugin.py:4: error: Library stubs not installed for "toml" (or incompatible with Python 3.8)
pylsp_black/plugin.py:4: note: Hint: "python3 -m pip install types-toml"
pylsp_black/plugin.py:4: note: (or run "mypy --install-types" to install all missing stub packages)
pylsp_black/plugin.py:4: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
tests/test_plugin.py:8: error: Library stubs not installed for "pkg_resources" (or incompatible with Python 3.8)
tests/test_plugin.py:8: note: Hint: "python3 -m pip install types-pkg_resources"
Found 2 errors in 2 files (checked 6 source files)