use pypy for tox

appveyor.yml

@@ -8,6 +8,12 @@ environment:
 init:
   - ps: echo $env:TOXENV
   - ps: ls C:\Python*
+  - ps: |
+      if ($env:TOXENV -match 'pypy') {
+        $env:TOX_PYTHON = pypy
+      } else {
+        $env:TOX_PYTHON = C:\Python35\python
+      }
 install:
   # install pypy using choco (redirect to a file and write to console in case
   # choco install returns non-zero, because choco install python.pypy is too
@@ -18,13 +24,11 @@ install:
   - pypy --version
 
   # Upgrade virtualenv for e.g. more-itertools to be handled properly.
-  - C:\Python35\python -m pip install -U virtualenv==16.7.2
-  - C:\Python35\python -m pip install tox==3.9
+  - "%TOX_PYTHON% -m pip install -U virtualenv==16.7.2"
+  - "%TOX_PYTHON% -m pip install tox==3.9"
 
 test_script:
-  - C:\Python35\python -m tox --version
-  - C:\Python35\python -m tox --showconfig -e %TOXENV%
-  - C:\Python35\python -m tox -v
+  - "%TOX_PYTHON% -m tox -v"
 after_test:
   - ls C:\projects\pytest-cov\.tox\pypy-t310-c45
   - ls C:\projects\pytest-cov\.tox\pypy-t310-c45\bin