Change subclassing behavior to Python standard

[normanrz]

The current implementation makes it very hard to discern objects of pathlib.Path from objects of UPath (and its subclasses).

upath = UPath("gcs://bucket")
path = pathlib.Path("/tmp")

assert isinstance(upath, UPath)  # true, which is expected
assert isinstance(upath, pathlib.Path)  # also true, which is expected, because UPath is a subclass of pathlib.Path

assert isinstance(path, pathlib.Path)  # true, which is expected
assert isinstance(path, UPath) # also true, but NOT expected

This is due to the fact that UPath overwrites isinstance and issubclass checks in a meta class.

In my user code, I currently discern between the two based on the existence of the _kwargs attribute.

if hasattr(path, "_kwargs"):
    upath = cast(UPath, path)
    ...

I think the current behavior is surprising and not very pythonic.

In this PR, I remove the multi-subclassing of UPath. This was unneccassary, because _flavour can be implemented directly in UPath and pathlib.Path is a subclass of pathlib.PurePath itself. With that, I could also remove the meta classes.
This PR also makes upath play more nicely with type checkers such as mypy or pylance.

[jstriebel]

Nice, I think valid isinstance checks are quite helpful here! Returning the parent class in new might still lead to confusion, but I think the current handling is more appropriate (see also the comment). Besides that I just have one minor nitpick:

[jstriebel]

PS: I assume that if issubclass(cls, UPath): in __new__ must always be True, no?

[normanrz]

PS: I assume that if issubclass(cls, UPath): in __new__ must always be True, no?

Theoretically, you can do UPath.__new__(pathlib.Path, ".") as in https://github.com/fsspec/universal_pathlib/blob/main/upath/tests/test_core.py#L97. But I don't think that is something that should be done.

[normanrz]

@andrewfulton9 Are you ok with merging this? It kinda reverts #29, but I think it provides a more standard-compliant behavior.

[andrewfulton9]

@normanrz, yeah, this looks good to me. I'll go ahead and merge and close it out.