pyproject.toml, pyrightconfig.json: Require @override

Add mypy's explicit-override error code and pyrightconfig's
reportImplicitOverride "error" directive. This requires all methods
that override base class methods to be decorated with @override from
typing. The change complements the previous commit, which added
exactly that decorator to all relevant methods.

Signed-off-by: Jan Lindemann <jan@janware.com>

conf/project/pyrightconfig-base.json

@@ -2,5 +2,6 @@
   "typeCheckingMode": "basic",
   "pythonPlatform": "Linux",
   "useLibraryCodeForTypes": true,
-  "verboseOutput": false
+  "verboseOutput": false,
+  "reportImplicitOverride": "error"
 }