extra/stbt_checker.py: Compatibility fix for pylint <= 0.25

Symbolic identifiers (names) for pylint messages were added in pylint
version 0.26:
https://bitbucket.org/logilab/pylint/commits/b9dab5059b81#Lutils.pyF129

In pylint 0.25 and earlier, these message definitions expected a tuple
with 2 elements, not 3. pylint 0.26 still accepts the 2-element form for
backwards compatibility. Ubuntu 12.04, for example, still carries pylint
0.25.