doctor: use -V for python version

`python --version` doesn't work in some (very) old versions of
python. While I doubt this is going to come up very often, it's
theoretically possible someone has an old python first in their
path. (python -V works on all versions I'm aware of, including
python3, and comes before --version in the --help anyway.)

Also skips the warning in case the regexp matches nothing, which
probably shouldn't happen. But if it does we shouldn't produce the
wrong warning.

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