When assert_queries() reports mismatches, pytest would show stack

traces going into assert_queries(), with all the logic and docs shown.

This made for long and annoying stack traces.
pytest, it turns out, has a handy __tracebackhide__ = True flag you

can set in any particular traceback frame context to prevent it from

showing up in stack traces. This needs to be as high up as possible to

take effect.
Unfortunately, this can't merely be set in assert_queries() itself or

in a decorator applying to assert_queries(), as the code needs to run

in the context manager's __exit__() function. So instead, we need to

return an explicit context manager instance that sets it at the right

time.
We now dynamically construct and return a context manager that sets this

at the right time. Right now, all this is constructed within

assert_queries(). It may be worthwhile breaking this and the other

functions out into a dedicated and more public class down the road, but

it's unclear at this time if there's much real-world value in doing

that.

All unit tests pass.
Verified that query assertion results appeared with the assert_queries() call.