Replace python with python3 in AuTest

[masaori335]

Fix #6386

Ran below command

find . -name "*.py" | xargs sed -i '' s/"python "/"python3 "/g

[ywkaras]

Maybe we should add this to the beginning of autest.sh?

if [[ $(which python) != "" ]]; then
  if [[ $(python --version) != Python?3*  ]]; then
    echo "Must use Python version 3 only" >&2
    exit 1
  fi
fi

[masaori335]

@ywkaras IIUC, test-env-check.sh is checking the version & requires 3.5+.

[masaori335]

I'm going to change to use sys.executable instead of python or python3 as @dragon512 recommended on slack.

[ywkaras]

What I'm suggesting is different, that the Au test framework require that the python command run python version 3.x.

[calavera]

I like consistency. I think @ywkaras suggestion can be added in another PR, but this looks necessary anyways.

[zwoop]

Cherry-picked to v9.0.x branch.

[zwoop]

Fwiw, I think a better approach would be to 1) enforce python3 by autest (as suggested above) and 2) make sure that any subsequent invocations of python to exec() the same python, which then properly enforces the same python version.