Install MyPy stubs

They were unbundled with MyPy 0.9.0 release:
https://mypy-lang.blogspot.com/2021/06/mypy-0900-released.html

Author: haplo

setup.cfg

@@ -21,7 +21,7 @@ python_requires = >= 3.6
 pylsp = pylsp_black = pylsp_black.plugin
 
 [options.extras_require]
-dev = isort>=5.0; flake8; pytest; mypy; pytest
+dev = isort>=5.0; flake8; pytest; mypy; pytest; types-pkg_resources; types-toml
 
 [flake8]
 max-line-length = 88