fix datetime types in ItemSearch

[philvarner]

Related Issue(s): #189

Description:

• fix datetime types in ItemSearch

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #227 (d21cc0e) into main (49de56d) will decrease coverage by 0.18%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
- Coverage   83.61%   83.42%   -0.19%     
==========================================
  Files          10       10              
  Lines         708      712       +4     
==========================================
+ Hits          592      594       +2     
- Misses        116      118       +2     

Impacted Files	Coverage Δ
pystac_client/stac_api_io.py	84.54% <83.33%> (ø)
pystac_client/client.py	81.25% <85.18%> (-1.65%)	⬇️
pystac_client/item_search.py	91.90% <92.98%> (ø)
pystac_client/collection_client.py	76.74% <100.00%> (ø)

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