fix item search types

[philvarner]

Related Issue(s): #189

Description:

• fix typing issues in ItemSearch

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #228 (eb73de0) into main (43b4325) will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main     #228   +/-   ##
=======================================
  Coverage   83.42%   83.42%           
=======================================
  Files          10       10           
  Lines         712      712           
=======================================
  Hits          594      594           
  Misses        118      118           

Impacted Files	Coverage Δ
pystac_client/item_search.py	91.90% <66.66%> (ø)

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