[ci] Add Python 3.7 in Appveyor build matrix

[olemis]

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 ).

[olemis]

@stdevPavelmc to work on this one you are gonna need skycoin/skycoindev-python:develop Docker image.

[stdevYuniers]

It has been solved on this appveyor file.

But, I have not finish #17 yet.

[stdevMac]

@olemis this is not a duplicated of #109 , i think this is just an edge of #109