Add typesafety checks

[ap--]

Close #210

This PR adds typesafety checks for the UPath class interface. Tests all public attributes / method return types.

currently missing (can be done in another PR):

• [ ] __fspath__
• [ ] __bytes__
• [ ] _scandir
• [ ] link_to (on <3.12)
• [ ] walk (on >=3.12)

Ideally, the type checks should also be done for subclasses (-> future PR).