Some Python-specific modifications

`railscasts` works great for Python, too, and I've added some Python-specific changes.

* the colors for the `pythonBuiltin` syntax match is removed from the "Ruby" section.  The default highlight group (given in `syntax/python.vim` should be good enough.  Even better (IMHO), one could highlight this syntax match as `Constant`, as I've done in congma/vim-syntax-python@786d9f7, which I believe is the correct place to go...
* `pythonExceptions` (Python's built-in exception classes) are highlighted with a light-ish warm color.
* Python doctests are highlighted with bluish cool colors, so that they are not in conflict withsurrounding docstrings, but still stand out somewhat.

Hope this be useful ;)