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 issues that when solved may improve the situation:

• Unable to specify a default value for a generic parameter python/mypy#3737
  This would make overloading unneded

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