Instantiate correct [R]JP subclasses

[handrews]

Fixes #103 (the rest of it).

Always return an instance of the subclass where appropriate. For RelativeJSONPointer, add a class property for the matching JSONPointer subclass.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.09% ⚠️

Comparison is base (e6f71d0) 92.73% compared to head (4d00a43) 92.65%.

❗ Current head 4d00a43 differs from pull request most recent head c4b691f. Consider uploading reports for the commit c4b691f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
- Coverage   92.73%   92.65%   -0.09%     
==========================================
  Files          23       23              
  Lines        2052     2041      -11     
  Branches      435      431       -4     
==========================================
- Hits         1903     1891      -12     
  Misses         97       97              
- Partials       52       53       +1     

Files Changed	Coverage Δ
jschon/jsonpointer.py	99.36% <100.00%> (+0.03%)	⬆️

... and 2 files with indirect coverage changes

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[handrews]

I'm going to take this down and do it as part of a comprehensive approach as is being discussed in #108

[handrews]

I'm re-opening this as its own PR to finish #103.

As discussed in #108 (reply in thread), I no longer think the class attribute approach is good for the rest of the library. I also no longer have immediate need to subclass these, but they should be subclassable (as this PR implements with cls() and type(self)() construction), and connecting paired subclasses and optional exception classes is straightforward for this self-contained area.