typing improvements

[philvarner]

Related Issue(s):

• Add typing info to all code to allow enabling mypy #189
• Providing a non-StacApiIO instance when creating a client will cause many methods to throw exceptions #186

Description:

• add several type declarations

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #225 (c49e2a8) into main (49de56d) will decrease coverage by 0.16%.
The diff coverage is 84.78%.

@@            Coverage Diff             @@
##             main     #225      +/-   ##
==========================================
- Coverage   83.61%   83.45%   -0.17%     
==========================================
  Files          10       10              
  Lines         708      713       +5     
==========================================
+ Hits          592      595       +3     
- Misses        116      118       +2     

Impacted Files	Coverage Δ
pystac_client/stac_api_io.py	84.54% <80.00%> (ø)
pystac_client/item_search.py	91.90% <83.33%> (ø)
pystac_client/client.py	81.48% <86.36%> (-1.42%)	⬇️
pystac_client/collection_client.py	76.74% <100.00%> (ø)

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