Use root _stac_io, regex for conformance

[matthewhanson]

Related Issue(s): #
Resolves #93

Description:
This PR changes conformance class checking to use regular expressions.

Also, changes calls to _stac_io in CollectionClient rather than requiring it's own _stac_io class (which went against how it works in PySTAC)

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #97 (d0028ff) into main (c9b117d) will decrease coverage by 1.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
- Coverage   77.55%   75.88%   -1.67%     
==========================================
  Files           9        9              
  Lines         539      535       -4     
==========================================
- Hits          418      406      -12     
- Misses        121      129       +8     

Impacted Files	Coverage Δ
pystac_client/collection_client.py	88.23% <100.00%> (+0.73%)	⬆️
pystac_client/conformance.py	100.00% <100.00%> (ø)
pystac_client/stac_api_io.py	76.63% <100.00%> (-8.13%)	⬇️

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