Only build wheels on supported CPython versions.

PyPy 3 wheels were failing to build on macOS.

Author: aaugustin

.appveyor.yml

@@ -6,7 +6,7 @@ skip_branch_with_pr: true
 
 environment:
 # websockets only works on Python >= 3.6.
-  CIBW_SKIP: cp27-* cp33-* cp34-* cp35-* pp27-*
+  CIBW_BUILD: cp36-* cp37-* cp38-*
   CIBW_TEST_COMMAND: python -W default -m unittest
   WEBSOCKETS_TESTS_TIMEOUT_FACTOR: 100
 

.travis.yml

@@ -1,7 +1,7 @@
 env:
   global:
   # websockets only works on Python >= 3.6.
-    - CIBW_SKIP="cp27-* cp33-* cp34-* cp35-* pp27-*"
+    - CIBW_BUILD="cp36-* cp37-* cp38-*"
     - CIBW_TEST_COMMAND="python3 -W default -m unittest"
     - WEBSOCKETS_TESTS_TIMEOUT_FACTOR=100