Make `str_to_interval` not return a tuple for single-value input

[avbentem]

Related Issue(s):

• 688

Description:

This is a simple fix to return a single datetime object if a single value (rather than an interval) is given. This is already supported by DateTimeType:

https://github.com/stac-utils/stac-fastapi/blob/f815c236cde78a3442fe18c103f43ee1db668872/stac_fastapi/types/stac_fastapi/types/rfc3339.py#L15-L20

It then also changes the value of the datetime argument as passed to get_search, which so far is passed a tuple for a single-value datetime as well.

PR Checklist:

• [x] pre-commit hooks pass locally
• [x] Tests pass (run make test)
• [x] Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• [x] Changes are added to the CHANGELOG.

[vincentsarago]

I wrongly fixed this in #697, your PR is a better fix 🙏