Proposed warning: `assert <generator_expression>`

For example: ```python assert (result[i] == expected[i] for i in range(len(expected))) assert (result[key] == expected[key] for key in result.keys() | expected.keys()) ``` bugbear could warn and suggest adding `all()`. Note that even an empty generator expression is truey: ```python assert (False for _ in range(0)) ``` so it's fine that bugbear doesn't statically know the length of the generator. There are a couple of similar warnings: * F631 is a similar rule for `assert <tuple>`. * Proposal (2) in #440 is a similar rule for `assert <constant>`.