* "_0" is a valid identifier!

More specifically, they are "built-in constants":

  https://docs.python.org/2/library/constants.html#built-in-consts

(I don't know how to create an empty group so I just put back "import" into
it.)

https://www.python.org/dev/peps/pep-0492/

The 'yield from' statement was introduced in Python 3.3. Reported by
Elizabeth Myers.

The option is disabled by default and when enabled highlight shebang and
coding file headers as comments instead of special characters.

Imports work like includes so they should use the relevant group-name.

I also renamed pythonPreCondit to pythonImport since it's specific to
that type of "preprocessor" statement.

Turning syntax off and back on is more than what's necessary to reload the syntax for the current buffer.

Merge Python 3 and Python 3 script versions into
the single python.vim script