Remove pystac

[jonhealy1]

Pystac is only used in one place - stac-fastapi.types.rfc3339.py - on one line of code using this method - from pystac.utils import datetime_to_str. It should probably be removed.

[jonhealy1]

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

Lines 125 to 127 in add05de

	def now_to_rfc3339_str() -> str:
	"""Return an RFC 3339 string representing now."""
	return datetime_to_str(now_in_utc())