adds python 3.7 compatibility

[benjamin-salchow]

I upgraded to Python 3.7 and I could not run your plug-in.

I got the following error: "Plugin Detailed Progress Plugin (0.1.4) is not compatible to Python 3.7.3 (compatibility string: >=2.7,<3)."

So I just added the plugin_pythoncompat information and run this plugin. Now it works like a charm with Python 3.7.3 and I had no issue with my OctoPrint setup.

[tpmullan]

I added this to my fork, and will test it for my next release: https://github.com/tpmullan/OctoPrint-DetailedProgress