Fix intervals

[gadomski]

Related Issue(s):

• Closes issues with datetime open interval validation #357

Description:

Add some additional checks and fixes to the datetime parsing and interval construction.

PR Checklist:

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

[codecov-commenter]

Codecov Report

Base: 85.18% // Head: 85.30% // Increases project coverage by +0.11% 🎉

Coverage data is based on head (230d168) compared to base (e45c68d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
+ Coverage   85.18%   85.30%   +0.11%     
==========================================
  Files          11       11              
  Lines         790      796       +6     
==========================================
+ Hits          673      679       +6     
  Misses        117      117              

Impacted Files	Coverage Δ
pystac_client/item_search.py	92.69% <100.00%> (+0.12%)	⬆️

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