[appveyor.yml] set tox testing to environment specific Py interpreters

Author: chrissimpkins

appveyor.yml

@@ -2,15 +2,19 @@ environment:
   matrix:
     - JOB: "3.5 64-bit"
       PYTHON_HOME: "C:\\Python35-x64"
+      test_script: "tox -e py35"
 
     - JOB: "3.6 64-bit"
       PYTHON_HOME: "C:\\Python36-x64"
+      test_script: "tox -e py36"
 
     - JOB: "3.7 64-bit"
       PYTHON_HOME: "C:\\Python37-x64"
+      test_script: "tox -e py37"
 
     - JOB: "3.7 32-bit"
       PYTHON_HOME: "C:\\Python37"
+      test_script: "tox -e py37"
 
 install:
   # If there is a newer build queued for the same PR, cancel this one.
@@ -37,7 +41,3 @@ install:
   - "python -m pip install tox"
 
 build: false
-
-test_script:
-  # run tests
-  - "tox"