Improve the datetime regex

[gadomski]

Related Issue(s): #57

Description: Improves the datetime regex. Includes a test with a lot of funky datetimes.

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #59 (e6dfb22) into main (2aa5ecf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   85.48%   85.48%           
=======================================
  Files          13       13           
  Lines         634      634           
=======================================
  Hits          542      542           
  Misses         92       92           

Impacted Files	Coverage Δ
pystac_client/item_search.py	80.74% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2aa5ecf...e6dfb22. Read the comment docs.