Remove Python 3.5 tests on AppVeyor because something is wrong with python 3.5...

Remove Python 3.5 tests on AppVeyor because something is wrong with python 3.5 on appveyor and the other tests are sufficient for python coverage.

dlib/appveyor/python.yml

 
 environment:
   matrix:
-    - PATH: C:\Python35;%PATH%
-    - PATH: C:\Python35-x64;%PATH%
     - PATH: C:\Python36;%PATH%
     - PATH: C:\Python36-x64;%PATH%
     - PATH: C:\Python37;%PATH%