dib-lint: match text/x-script.python
dib-lint: match text/x-script.python

It seems libmagic changed it's Python output to
"text/x-script.python", which I see on Fedora.  Handle this too so we
detect Python files correctly.

[1] https://github.com/file/file/commit/eb373e431ccfeedfbcf497e4da07571d43bdb9f2

Change-Id: I35992c70523a8f2bc5efff2e5167ed1ac1514d34

openstack/diskimage-builder,openstack/diskimage-builder