Add Python 3.7 compatibility

[borupdaniel]

It seems like it might be good to add Python 3.7 compatibility at some point. I'm guessing this hasn't been looked at yet. I did some quick using the code from PR #11 — we should probably get that PR finished first, then move on to this issue, if it's something we want to pursue.

To test things out I cloned my normal development conda environment and installed Python 3.7.3, then installed gpi from conda with --no-deps and re-added packages that the python upgrade had removed. GPI seems to load okay, but nodes using C++ with PyFI don't load — not sure if I just need to recompile them, or if there's something else going on. I haven't tested it with anything except a simple Shapes -> ImageDisplay network yet.

Here's a quick screengrab comparing my gpidev (Python 3.6) environment with the new environment that seems to work for basic things.

[borupdaniel]

@aganders3, I see you're already looking ahead to Python 3.8 in #16, so feel free to combine these as appropriate.

[borupdaniel]

Just to update, it looks like this is coming from the conda dependencies for 1.0.5 and may stem from requiring PyQt4, which is only compatible up through Python 3.6:

GPI 1.0.5 dependencies:

PyQt4 package info request on builds for Python 3.6 (1) and 3.7 (none)