Stop explicitly depending on python 2
This PR revises our previous policy of officially only supporting and testing with python 2 in the CI environment to instead test with python 3. It also changes the defaults to python 3 in our various scripts (usually, by way of `python` rather than `python3` to preserve compatibility with systems that do not have a python 3 available).
The effect of this is that we expect all new patches to support python 3 (and will test as such). We explicitly also expect that patches support python 2.7 as well -- and test as such, though only on one builder. This is intended as a temporary, though likely long-lived, measure to preserve compatibility while looking towards the future which is likely to be a python 3 only world. We do not at this point set a timeline for when we'll drop support for python 2.7; it's plausible that this is months or years into the future, depending on how quickly the ecosystem drops support and how painful it is for us to maintain that support over time.
Closes#65063 (as far as I can tell; please file explicit and separate issues or PRs if not).
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