Case sensitive support for custom log levels

[paveldedik]

To quote @radiophysicist

the problem is that logging doesn't prohibit to register custom log-level with name LeVeL and you try to look it for as LEVEL instead

i.e. we probably need update the method _get_level_value to account for custom log-levels with weird casings:

structlog-sentry/structlog_sentry/__init__.py

Lines 181 to 193 in db2b9bb

	def _get_level_value(level_name: str) -> int:
	"""Get numeric value for the log level name given."""
	try:
	# Try to get one of predefined log levels
	return getattr(logging, level_name)
	except AttributeError as e:
	# May be it is a custom log level?
	level = logging.getLevelName(level_name)
	if isinstance(level, int):
	return level
	# Re-raise original error
	raise ValueError(f"{level_name} is not a valid log level") from e