Typing workarounds and list of relevant mypy issues

[janluke]

Cloup currently uses overloading (and a lot of ugly signature replication) to work around mypy limitations
These are a couple of relevant Mypy/Pyrefly issues that when solved may improve the situation:

Open issues

• Unable to specify a default value for a generic parameter python/mypy#3737

  This would make overloading unnecessary.

• PEP 728

  • Mypy: Add support for TypedDict extra_items= and closed= (PEP 728) python/mypy#18176
  • Pyrelfy: Add support for PEP 728 (TypedDict with extra_items= and closed=) facebook/pyrefly#946

  This would allow for better types for the parameter decorators in the case where a custom class is provided. Currently we are forced to annotate **attrs: Any, while **attrs should be a typed dict that expands OptionKwargs/ArgumentKwargs.

Closed issues

• Allow using TypedDict for more precise typing of **kwds python/mypy#4441
  This allows to avoid replication of kwargs annotations.

  UPDATE: we now use this for parameter decorators. We still don't for command decorators.