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)
count if None count of messages is not asserted, otherwise the number of deprecation messages have to equal count.