replace ciso8601 with pyiso8601

[geospatial-jeff]

Related Issue(s):

• Closes ciso8601 as an optional dependency #447

Description:
Replaces the ciso8601 dependency with pyiso8601, following validation logic recommended in the stac api implementation guide:

• Upper case the date string.
• Match against the regex ^(\d\d\d\d)\-(\d\d)\-(\d\d)(T|t)(\d\d):(\d\d):(\d\d)([.]\d+)?(Z|([-+])(\d\d):(\d\d))$
• Parse with pyiso8601.

The date parsing unittests are quite good and all pass with pyiso8601.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[geospatial-jeff]

Need to add types test cases to ci