Make pystac pass User-Agent to work around urllib being blocked

[jsignell]

Related Issue(s):

• Closes Item validation failing to resolve json schema endpoint #1575

Description:

Probably best to explicitly include a user agent in the header to avoid this in the future.

PR Checklist:

• Pre-commit hooks pass (run pre-commit run --all-files)
• Tests pass (run pytest)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.38%. Comparing base (e973546) to head (78d7a89).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pystac/stac_io.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1576   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files          55       55           
  Lines        8374     8374           
  Branches      965      965           
=======================================
  Hits         7736     7736           
  Misses        453      453           
  Partials      185      185           

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[jsignell]

I'm just rewriting the cassetts and adding a note to the changelog