Check coverage across python runtimes

Recently, we have been adding conditionals that perform different logic depending on the python version or dependency versions (https://github.com/googleapis/gapic-generator-python/pull/2477, https://github.com/googleapis/gapic-generator-python/pull/2472)

Individual coverage checks aren't aware of the big picture, so individual runs often fail the coverage check, unless we add `pragma: no cover` statements

We should find a way to fix this at some point, to make sure we don't make a habit of adding `no cover` and losing required coverage