* An example to use this style: tarfaret library

 * It is recommended to override it in colorschemes.

 * Builtin-functions are highlighted only when used as global functions.
 * Annotations in type-only variable definitions and assignment statements are not supported yet due to potential conflicts with dictionary expressions.
 * Uses a custom syntax highlight def "Optional" (in my setup, it applies italic style.)
 * Taken from purpleP/python-syntax commits and customized.

 * Make them highlighted only when used as in the global namespace. (e.g., `id` in `something.id` is not highlighted)
 * Also add self and cls to builtin objects

 * "_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