Use abc.ABC as baseclass for AnyPath to fix issubclass behavior

[chbehrens]

Following @jayqi's suggestion in #247, this PR lets AnyPath inherit from abc.ABC and sets its subclasses via ABC.register. It also adds a test to check the bug filed in #246.

Fixes #246

[pjbull]

Hm @chbehrens, looks like mypy and ABC have some rough edges. See python/mypy#2922, python/mypy#5374, https://stackoverflow.com/questions/69330256/how-to-get-an-abstract-dataclass-to-pass-mypy

Ironically, it looks like the metaclass implementation might pass mypy (see python/mypy#1459 (comment))

I'm ok with a # type: ignore on the line that is erroring. That work for you @jayqi?

[jayqi]

I'm also fine with a # type: ignore.

I was also looking at python/mypy#5374 and it seems like it's possible this is addressed very recently in mypy (just 5 days ago) by python/mypy#13398. That PR was merged but is not released yet, so I think we'll potentially look at revisiting once the next release of mypy is published.

[chbehrens]

Ok, I added the # type: ignore and will try to keep it in mind.

[pjbull]

Thanks @chbehrens! You can ignore the failure for now, Github Actions is having an outage:
https://www.githubstatus.com/

I'll rerun these later once they stabilize the service

[pjbull]

Changes look good, thanks @chbehrens! I'm going to move this to a local branch to run the live server tests.