Add python3.7

[aselus-hub]

py3.7 has been released and there's no snap for it, would love to have it added.

[abulka]

I too am struggling with Python 3.7 and snap, specifically a wxPython 4.0.4 app I'd like to get into the snap store. Building with core18 rejects my reference to a wxPython wheel built with Python 3.7 but builds and runs ok with a reference to a Python 3.6 wxPython wheel.

I wonder if I can specify Python 3.7 somehow in the snapcraft.yaml, or do I need a reference to another snap like this one (if ever a Python 3.7 version of it appears)?