Check coverage across python runtimes

[daniel-sanche]

Recently, we have been adding conditionals that perform different logic depending on the python version or dependency versions (googleapis/gapic-generator-python#2477, googleapis/gapic-generator-python#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

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.