fix conformance class checking

[philvarner]

Related Issue(s):

• client.get_collections should check for other conformance classes #185
• Use of Query Extension does not validate Query conformance class is advertised #221
• Add typing info to all code to allow enabling mypy #189

Description:

• Incorrect conformance class was being checked for Collections
• most Item Search extensions were not being checked.
• fix typing definitions so modified code type checks correctly

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #223 (2bfd60b) into main (6a1295e) will increase coverage by 0.06%.
The diff coverage is 91.89%.

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   83.47%   83.54%   +0.06%     
==========================================
  Files          10       10              
  Lines         702      711       +9     
==========================================
+ Hits          586      594       +8     
- Misses        116      117       +1     

Impacted Files	Coverage Δ
pystac_client/client.py	82.05% <75.00%> (+0.10%)	⬆️
pystac_client/collection_client.py	76.74% <100.00%> (ø)
pystac_client/conformance.py	100.00% <100.00%> (ø)
pystac_client/item_search.py	91.92% <100.00%> (+0.07%)	⬆️

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