This introduces TestCase.assertWarnings(), which takes a list of

dictionaries representing warnings and compares against them. Like our

version of assertWarns(), it can compare against classes and then

optionally messages. This can be extended for other capabilities, like

native regex matching, if needed.
This can be used both for asserting that there are warnings (by

specifying one or more items in the list) or no warnings (by specifying

an empty list).
Both TestCase.assertWarns() and assertNoWarnings() now wraps this,

helping us manage just one set of logic.
TestCase.assertWarns() is pending a deprecation phase It's

incompatible with Python's own version of assertWarns(), and as such

we should probably move off of it. For now, it's not documented as

deprecated, but it does have a big comment about it. We'll want to

figure that out at some point, but not today.

All unit tests pass.
Tested the multiple warning matching in an in-progress change.