Add [mypy] extra dependency set for type checking

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

setup.cfg

@@ -47,6 +47,9 @@ lint =
     flake8-builtins
     flake8-isort
     flake8-pytest-style
+mypy =
+    mypy
+    types-xmltodict ~= 0.12
 
 [options.data_files]
 share/doc/obs-proxy/examples = proxy.conf