Use isoparse instead of parse for datetimes

[duckontheweb]

Related Issue(s):

• Closes pystac.read_file returns temporal extents with tzinfo=tzlocal() depending on the environment. #819

Description:

Updates pystac.utils.str_to_datetime to use dateutil.parser.isoparse instead of dateutil.parser.parse to ensure UTC timestamps stay in UTC even if the system timezone is set to UTC as well. There were also a couple of places in the Item and Collection classes where we were using dateutil.parser.parse directly instead of using pystac.utils.str_to_datetime. These have all been updated to use str_to_datetime.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• 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.

[duckontheweb]

@TomAugspurger I verified that this fixes the original issue in pystac.read_file as well:

$ TZ=UTC python -c 'import pystac; print(pystac.read_file("https://planetarycomputer.microsoft.com/api/stac/v1/collections/sentinel-2-l2a").extent.temporal.intervals[0][0].tzinfo)'
tzutc()

[TomAugspurger]

Thanks!