Minor improvements to convertors

[JayH5]

• typing: Make convertors generic over their return types.
• PathConvertor can inherit from StringConvertor
• Use r-strings for regex pattern literals.

[JayH5]

It's not really clear to me what the type of value is. Possibly it can also be T, but the semantics of to_string are not documented so I'm not sure.

[tomchristie]

Neat bit of work - thanks.

It's a bit of a mixed bag if the Generic typing is an improvement or not, since the implementation style of the way that convertors are used (create them, given the path pattern, use them to populate request.path_params) means that the typing information provided in (say) class FloatConvertor(Convertor[float]): doesn't actually ever become exposed anyplace.

Net result... we might just want to stick with simplicity and continue to under-specify the return type, as we currently do.

Not sure. 🤷‍♂️

[JayH5]

Thanks for the thoughtful review. I think I agree that this doesn't add terribly much except checking that the convertors return the expected type. Seems like we're all still figuring out when typing is most useful in Python.

I've reverted the change that made PathConvertor inherit from StringConvertor. I'll leave this for now but feel free to merge or close. Otherwise in a few days I'll close this. (The tests are failing now for some seemingly unrelated reason, though 😞)