multimethods: avoid calling caller_locals() in Python 3 (#13238)

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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