buildconfig only for python3

Author: mbucchi

.appveyor.yml

@@ -6,7 +6,8 @@ branches:
 environment:
   global:
     # only do for Python >= 3.6.
-    CIBW_SKIP: "cp27-* cp33-* cp34-* cp35-*"
+    CIBW_BUILD: "cp3?-*"
+    CIBW_SKIP: "cp33-* cp34-* cp35-*"
     CIBW_BEFORE_BUILD: "python -m pip install -r requirements.txt"
 
     TWINE_USERNAME: mabucchi