Noticed by y.ershov@mongodb.com: some scripts in dist, like s_clang_format, s_docs, s_function and s_all use simply "python" rather than "python3".

I think there are two ways of thinking - in the first way we could be to fix (some? all?) of them all to be python3.

For the second way, consider that many people use virtual environments when running python (it's a good way to have complete control over your python version and what modules are loaded). When a virtual environment is created, you decide if you want the "python" on your path to be "python2" or "python3".  If we want to standardize on using virtual environments, then we could leave everything as "python", and to be sure, put a  check in s_all that "python" is the expected version.