Make ropevim python3 compatible (for Debian)

[bgribble]

For some time, Debian's vim has been built with +python3 -python. See this stackexchange question with an answer from the Debian vim maintainer. It appears to be a packaging issue rather than an ideological or technical choice.

I spent an hour or so last night trying to make ropevim work with either python 2 or 3, and made some headway but it's not there yet. I'll put up a PR if I get something working. If someone else wants to do it first be my guest!

[fdjcomp]

this is the reasaon packages get a bad reputation
I had a similar frustrating experience with no real luck to get it working
much of the pyhon code seems to be for pyhon2 (and thus fails in vim debian)
a warning to this effect is the minimum requirement

[soupytwist]

I think python3 support in rope is in a much better state than it was. Can you share any specific errors? I'm curious what's lacking python3 support, not obvious from a quick look over the code.

[mcepl]

I am actually running ropevim with python3 (3.7.3) in neovim (almost 0.5.0), and it works just fine. The only problem is variable :ropevim_prefer_py3 which has to be set. I will fix that momentarily.