Make compatible with Neovim

[galou]

Reverts 91afcf8f623c65a99dfb678b50bb8cb438fe6a10 because it made atp incompatible with NeoVim, unfortunately. pythonx is also only available in recent Vim versions.

Please consider merging at least the first commit that solves mixing of tabs and spaces, generating a syntax error with Python 3.

A small contribution to express my gratitude.

[galou]

I forgot to mention that it doesn't work 100% with python3 because of Unicode error, among others, but most things seem to work.

[coot]

I haven't used atp for a very long time (it's just to heavy for my current latex work), and I stopped maintaining it due to time constraints; I encourage you to make a clone for neovim.