Make str_to_interval not return a tuple for single-value input

[avbentem]

Related Issue(s):

• How to tell if search is for single datetime? (Should str_to_interval return a single value for that?) #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:

stac-fastapi/stac_fastapi/types/stac_fastapi/types/rfc3339.py

Lines 15 to 20 in f815c23

	DateTimeType = Union[
	datetime,
	Tuple[datetime, datetime],
	Tuple[datetime, None],
	Tuple[None, datetime],
	]

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:

• pre-commit hooks pass locally
• 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.

[vincentsarago]

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