Fixed: music21.mei.test_main.py Works Again

When I moved the tests (in commit e44cef3) I forgot to make changes to imports.
Now I've fixed the imports.
Refactoring: mei.__main__._articulationFromAttr()

The function previously used a try/except statement as a control structure, so
we could re-raise an exception if relevant. The function now uses an if
statement in a way that avoids raising exceptions we can solve, so it just
propagates.