appveyor: install tox from pypi, not git

The windows-specific bugfix we needed is now in the current Tox release,
so we no longer need pre-release code. Anyways the upstream git repo
moved (from bitbucket to github), so the install-from-bitbucket was
failing.

.appveyor.yml

@@ -39,8 +39,7 @@ matrix:
 
 install:
   - |
-    %PYTHON%\python.exe -m pip install -U pip virtualenv
-    %PYTHON%\python.exe -m pip install wheel "hg+https://bitbucket.org/hpk42/tox#egg=tox"
+    %PYTHON%\python.exe -m pip install -U pip virtualenv wheel tox
 
 # note:
 # %PYTHON% has: python.exe