fix typing in StacApiIO

[philvarner]

Related Issue(s): #189

Description:

• fix typing in StacApiIO class

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #229 (17209a0) into main (43b4325) will decrease coverage by 0.02%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
- Coverage   83.42%   83.40%   -0.03%     
==========================================
  Files          10       10              
  Lines         712      711       -1     
==========================================
- Hits          594      593       -1     
  Misses        118      118              

Impacted Files	Coverage Δ
pystac_client/item_search.py	91.90% <66.66%> (ø)
pystac_client/client.py	81.25% <100.00%> (ø)
pystac_client/stac_api_io.py	84.40% <100.00%> (-0.15%)	⬇️

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