scripts/checkpatch.pl: Only allow Python 3 interpreter

scripts/checkpatch.pl: Only allow Python 3 interpreter

Since commit ddf9069963 QEMU requires Python >= 3.5.

PEP 0394 [*] states that 'python3' should be available and
that 'python' is optional.

To avoid problem with unsupported versions, enforce the
shebang interpreter to Python 3.

[*] https://www.python.org/dev/peps/pep-0394/

Reported-by: John Snow <jsnow@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-2-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>