[ci] Add Python 3.7 in Appveyor build matrix

After #18 PySkycoin has support for almost every version of Python in use nowadays, except 3.7 . This version should be supported in Mac OS and GNU/Linux after closing #25 .

This is issue is for continuing this work until Python 3.7 be added successfully in Appveyor Windows builds ( after #17 ).

Start by locating the Appveyor configuration and reviewing its existing Python build matrix. Add Python 3.7 to the Windows builds, then run or inspect the Appveyor results to confirm the version builds successfully alongside the supported versions.

Written by the indexing model from the issue text.