repo: try to reexec self with Python 3 as needed

We want to start warning about Python 2 usage, but we can't do it
simply because the shebang is /usr/bin/python which might be an old
version like python2.7.

We can't change the shebang because program name usage is spotty at
best: on some platforms (like macOS), it's not uncommon to not have
a `python3` wrapper, only a major.minor one like `python3.6`.  Using
python3 wouldn't guarantee a new enough version of Python 3 anyways,
and we don't want to require Python 3.6 exactly, just that minimum.

So we check the current Python version.  If it's older than the ver
of Python 3 we want, we search for a `python3.X` version to run.  If
those don't work, we see if `python3` exists and is a new enough ver.
If it's not, we die if the current Python 3 is too old, and we start
issuing warnings if the current Python version is 2.7.  This should
allow the user to take a bit more action by installing Python 3 on
their system without having to worry about changing /usr/bin/python.

Once we require Python 3 completely, we can simplify this logic a bit
by always bootstrapping up to Python 3 and failing with Python 2.

We have a few KI with Windows atm though, so keep it disabled there
until the fixes are merged.

Bug: https://crbug.com/gerrit/10418
Change-Id: I5e157defc788e31efb3e21e93f53fabdc7d75a3c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253136
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

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