Add pytypes property to parameters to return Python type declaration

[philippjfr]

Decided to split out the less experimental part of https://github.com/holoviz/param/pull/636. This PR adds a pytypes property to all Parameters that returns a Python type declaration for a specific parameter. This can be used in a number of ways, e.g. for documentation but also eventually for a tool that can automatically generate types for Parameterized classes (which I had prototyped in #636).

[maximlt]

@philippjfr do you need this in 2.0?

[jbednar]

I'll be glad to have this mapping between Parameter types and py3 types, but I can't comment on whether this is the best way to achieve that. Up to @philippjfr .

[philippjfr]

I don't think it's ready, I want to revisit this as part of a more general plan for composite types as well as dataclass and pydantic integrations.