Strictly follow PEP3119 for AnyPath __subclasscheck__ implementation

[chbehrens]

The currently implemented subclass check for the AnyPathMeta class results in issubclass(AnyPath, AnyPath) returning False, opposed its expected behavior. With this PR I suggest to switch to the exact implementation shown in PEP 3119. It should resolve this problem while preserving the intended behavior with respect to CloudPath and pathlib.Path.

See #246

[chbehrens]

Sorry, I tested it before but messed it up during clean-up. Should be complete now.

[codecov-commenter]

Codecov Report

Merging #247 (30b02b0) into master (4cc0d33) will decrease coverage by 0.5%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #247     +/-   ##
========================================
- Coverage    95.0%   94.4%   -0.6%     
========================================
  Files          21      21             
  Lines        1324    1326      +2     
========================================
- Hits         1258    1253      -5     
- Misses         66      73      +7     

Impacted Files	Coverage Δ
cloudpathlib/anypath.py	100.0% <100.0%> (ø)
cloudpathlib/s3/s3client.py	92.9% <0.0%> (-2.7%)	⬇️
cloudpathlib/gs/gsclient.py	92.8% <0.0%> (-1.8%)	⬇️
cloudpathlib/cloudpath.py	93.5% <0.0%> (-0.5%)	⬇️

[jayqi]

I'm wondering if my original decision to use custom __subclasscheck__ and __instancecheck__ is the wrong approach here.

I believe another way of accomplishing what we want (that is also part of PEP 3119) is to have AnyPath subclass abc.ABC and use ABC.register. This would mean getting rid of our AnyPathMeta class since we wouldn't need it, and instead having

AnyPath.register(Path)
AnyPath.register(CloudPath)

I just did a little bit of light testing, and I think it should work equivalently and also resolve the issue of issubclass(AnyPath, AnyPath).

---

I'm lukewarm on the idea of copying the exact implementation for __subclasscheck__ and __instancecheck__ from PEP 3119. PEP 3119 describes those as "naively simple" from-scratch implementations. According to later in PEP 3119:

The call isinstance(x, C) first checks whether C.instancecheck exists, and if so, calls C.instancecheck(x) instead of its normal implementation. Similarly, the call issubclass(D, C) first checks whether C.subclasscheck exists, and if so, calls C.subclasscheck(D) instead of its normal implementation.

So that means by doing it this way with __subclasscheck__ and __instancecheck__, we're entirely responsible for the logic of isinstance and issubclass rather than leaning on any default logic.

[pjbull]

I like the implementation @jayqi laid out. @chbehrens would you be willing to take a hack at that version?

Could you also add a test to the subclass tests to check the bug you filed in #246?

[pjbull]

Resolved by #251