ci: AppVeyor: only install PyPy if used [skip travis]

Author: blueyed

appveyor.yml

@@ -18,10 +18,11 @@ 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
   # noisy)
-  - choco install python.pypy > pypy-inst.log 2>&1 || (type pypy-inst.log & exit /b 1)
-  - set PATH=C:\tools\pypy\pypy;%PATH% # so tox can find pypy
-  - echo PyPy installed
-  - pypy --version
+  - if not x%TOXENV:pypy=%==x%TOXENV% (
+      (choco install python.pypy > pypy-inst.log 2>&1 || (type pypy-inst.log & exit /b 1)) &&
+      set "PATH=C:\tools\pypy\pypy;%PATH%" &&
+      pypy --version
+    )
 
   # Upgrade virtualenv for e.g. more-itertools to be handled properly.
   # Pin it to work around https://github.com./tox-dev/tox/issues/1389.