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/templates/pyproject.toml

@@ -2,6 +2,8 @@
 
   {mypypath}
 
+  enable_error_code = "explicit-override"
+
 [tool.isort]
 
   lines_between_sections = 1