Python 3 metaclasses have a `__prepare__` method that lets us save the
class's dictionary before it is constructed. In Python 2 we had to walk
up the stack using our `caller_locals()` method to get at this. Using
`__prepare__` is much faster as it doesn't require us to use `inspect`.
This makes multimethods use the faster `__prepare__` method in Python3,
while still using `caller_locals()` in Python 2. We try to reduce the
use of caller locals using caching to speed up Python 2 a little bit.
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