Remove deprecated "is not" usage in maktaba.py (#233)
Remove deprecated "is not" usage in maktaba.py (#233)

Using "is" or "is not" operators with a string literal now produces a
SyntaxWarning in python 3.8
(https://docs.python.org/3/whatsnew/3.8.html#changes-in-python-behavior),
and the warning seems to cause some versions of neovim to blow up in
cases like #231.
google/vim-maktaba,malcolmr/vim-maktaba