Use one job for each tox env on AppVeyor

Some time ago when we first added support for testing pytest on AppVeyor,
jobs in a build would not start immediately one after the other, as if AppVeyor
would schedule jobs from other builds (projects) in its VMs. So it made sense
at the time to reduce the number of jobs.

I have noticed in other projects that this behavior has changed, and jobs
in a build now start one after the other. Having a separate list then improves
visibility when the build fails, because we can see at a glance the failing(s)
tox environment(s).

appveyor.yml

@@ -6,17 +6,23 @@ environment:
     # https://www.appveyor.com/docs/build-configuration#secure-variables
 
   matrix:
-  # create multiple jobs to execute a set of tox runs on each; this is to workaround having
-  # builds timing out in AppVeyor
-  - TOXENV: "linting,py26,py27,py33,py34,py35"
-  - TOXENV: "py27-pexpect,py27-xdist,py27-trial,py35-pexpect,py35-xdist,py35-trial"
-  - TOXENV: "py27-nobyte,doctesting,freeze,docs"
-  - TOXENV: "pypy"
-
-matrix:
-  allow_failures:
-    # pypy is disabled until #1963 gets fixed
+  - TOXENV: "linting"
+  - TOXENV: "py26"
+  - TOXENV: "py27"
+  - TOXENV: "py33"
+  - TOXENV: "py34"
+  - TOXENV: "py35"
   - TOXENV: "pypy"
+  - TOXENV: "py27-pexpect"
+  - TOXENV: "py27-xdist"
+  - TOXENV: "py27-trial"
+  - TOXENV: "py35-pexpect"
+  - TOXENV: "py35-xdist"
+  - TOXENV: "py35-trial"
+  - TOXENV: "py27-nobyte"
+  - TOXENV: "doctesting"
+  - TOXENV: "freeze"
+  - TOXENV: "docs"
 
 install:
   - echo Installed Pythons