fix: remove default argument now from _seconds_until_refresh

[jackwotherspoon]

_seconds_until_refresh which is used for background refresh strategy
(lazy refresh is unaffected!) should not have now as a default argument
due to Python only evaluating default arguments once at function definition.

Python's default arguments are evaluated only once when the function
is defined, not each time the function is called. (source)

Problematic code:

alloydb-python-connector/google/cloud/alloydb/connector/refresh_utils.py

Lines 29 to 53 in 14d6b1c

	def _seconds_until_refresh(
	expiration: datetime, now: datetime = datetime.now(timezone.utc)
	) -> int:
	"""
	Calculates the duration to wait before starting the next refresh.
	Usually the duration will be half of the time until certificate
	expiration.
	Args:
	expiration (datetime.datetime): Time of certificate expiration.
	now (datetime.datetime): Current time (UTC)
	Returns:
	int: Time in seconds to wait before performing next refresh.
	"""
	duration = int((expiration - now).total_seconds())
	# if certificate duration is less than 1 hour
	if duration < 3600:
	# something is wrong with certificate, refresh now
	if duration < _refresh_buffer:
	return 0
	# otherwise wait until 4 minutes before expiration for next refresh
	return duration - _refresh_buffer
	return duration // 2

Having now as a default argument means it is set once and then never
adjusted. So in subsequent calls to _seconds_until_refresh the duration
will just continue to grow larger and larger as it evaluates agains the same now

The result of this means that duration // 2 block is always being hit and
will continue to grow. It could grow to be greater than 3600 and lead to errors.

[enocom]

Probably should add a note that this only affects clients using the default refresh strategy and not lazy refresh.

[jackwotherspoon]

Probably should add a note that this only affects clients using the default refresh strategy and not lazy refresh.

good call, added a note to PR description

[enocom]

Fixes #357