Allows deprecated functions, classes, decorators in tests.

It acts as a contextmanager that can be used in with statements:

>>> with assert_deprecated("use cirq.x instead", deadline="v0.9"):
>>>     # do something deprecated

msgs messages that should match the warnings captured
deadline the expected deadline the feature will be deprecated by. Has to follow the format vX.Y (minor versions only)
allow_multiple_warnings if True, multiple warnings are accepted. Typically this should not be used, by default it's False.