Simplify appveyor.yml

I don't know what these PROFILE/PYTHON_VERSION environment variables are
supposed to be (or why PYTHON_VERSION is 3.6 for three different values
of PROFILE=py3{5,6,7}-conventions), but python -V reports the same "2.7.15"
version for all four of these jobs.

I see no reason to run the same set of tox jobs on the same Python
version four times (while incorrectly claiming they use different Python
versions).

appveyor.yml

@@ -1,23 +1,10 @@
 # AppVeyor CI settings (Windows Machine CI Tests)
 
-matrix:
-  allow_failures:
-    - PROFILE: py38
-
 environment:
   matrix:
     - PROFILE: py27-conventions
       PYTHON_VERSION: "2.7"
       TOXENV: "lint,docs"
-    - PROFILE: py35-conventions
-      PYTHON_VERSION: "3.6"
-      TOXENV: "lint,docs"
-    - PROFILE: py36-conventions
-      PYTHON_VERSION: "3.6"
-      TOXENV: "lint,docs"
-    - PROFILE: py37-conventions
-      PYTHON_VERSION: "3.6"
-      TOXENV: "lint,docs"
     - PROFILE: py27
       PYTHON_VERSION: "2.7"
       TOXENV: "py27,py27-datetime"