Add mypy to dev dependencies

.gitignore

 # Python egg metadata, regenerated from source files by setuptools.
 /*.egg-info
 /*.egg
+
+# Type checking cache
+.mypy_cache

setup.py

     },
     install_requires=[],
     extras_require={
-        "dev": ["black", "pylint"],
+        "dev": ["black", "pylint", "mypy"],
         "server": ["Flask", "SQLAlchemy", "Flask-SQLAlchemy", "alembic", "uwsgi"],
         "client": ["PySide2", "qdarkstyle>=2.6.0", "requests", "simpleaudio"],
         "osk": ["dbus-python"],