The match and case soft keywords are shown in bold when they are
keywords, and not when they are not.

The underscore soft keyword is ignored, because it is harder to get
right, and because it doesn't look that much different in bold anyway.

This need 3.10.0b3 (not yet released) to fully pass.

Every statement-level ast node should be tested. Annotated assignment
was missing.

Also, we don't need "exec" anymore, that was only for Python 2.

And: this is the 1000th test!

The code for the helper is uninteresting and long, and only makes it harder to
see what is going wrong.  Hide it.

https://docs.pytest.org/en/latest/example/simple.html#writing-well-integrated-assertion-helpers

Use the same rounding rules for the fail-under message that are used for totals
everywhere else, so that it won't say:

    total of 100 is less than fail-under=100

On Python 3.10, we were getting other warnings mixed into the warnings the tests
were looking for.  This change lets us only look at the CoverageWarnings.

The type-check is left over from Python 2 compatibility, we don't need
it anymore.