Remove mypy dependency from the install requires. (#154)

It's a peer dependency and only needed if you use the mypy plugin.
It's not needed for people not using mypy, and drags in a lot of weight.
People who want to use the mypy plugin will already have mypy installed
anyway.

Author: tasn

setup.cfg

@@ -28,7 +28,6 @@ project_urls =
 python_requires = >=3.6
 
 install_requires =
-    mypy>=0.800
     typing-extensions>=3.7.4
 
 [options.data_files]