Forked from vim-python/python syntax with a few changes:

• raise,assert moved from pythonStatement to pythonException
• = moved from pythonOperator to pythonStatement
• 'single quote strings' moved from pythonString to the new pythonSymbol, which extends Label
• in is in pythonOperator, except the in from for .. in, which is in pythonRepeat
• from will now correctly match pythonImport
• this has been added to pythonClass