Tokens are not properly immutable

[MetRonnie]

Description

#5769 sought to make Tokens instances immutable

cylc-flow/cylc/flow/id.py

Lines 120 to 124 in 98dc86a

	def __setitem__(self, key, value):
	raise Exception('Tokens objects are not mutable')
	def update(self, other):
	raise Exception('Tokens objects are not mutable')

However, this does not prevent

tokens.pop(key)
del tokens[key]
tokens.clear()
tokens.popitem()
tokens.setdefault(key, default)

and also there is this method that is used in a couple of places:

cylc-flow/cylc/flow/id.py

Lines 260 to 261 in 98dc86a

	def pop_token(self) -> Tuple[str, str]:
	"""Pop the lowest token.