language-specs: Add a content type for python3.lang
language-specs: Add a content type for python3.lang

Bug #100034 in shared-mime-info adds a new text/x-python3 content type
to distinguish Python 3 files from Python 2. Use that if available to
select python3.lang over python.lang.

See https://bugs.freedesktop.org/show_bug.cgi?id=100034.

https://bugzilla.gnome.org/show_bug.cgi?id=779468

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