eo.py: Add more typing info for module level strings

[schwehr]

Description:

Trivial addition of typing to module level strings

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• 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-commenter]

Codecov Report

Merging #377 (ec5d222) into main (e90f809) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #377   +/-   ##
=======================================
  Coverage   89.77%   89.77%           
=======================================
  Files          37       37           
  Lines        4677     4677           
=======================================
  Hits         4199     4199           
  Misses        478      478           

Impacted Files	Coverage Δ
pystac/extensions/eo.py	97.33% <100.00%> (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 e90f809...ec5d222. Read the comment docs.