[appveyor] Update supported Python versions

Remove tests for Python < 3.7 due to updated rdflib version.

Author: mpsonntag

appveyor.yml

@@ -4,9 +4,6 @@ image: Visual Studio 2017
 
 environment:
   matrix:
-    - PYTHON: "C:\\Python36"
-      PYVER: 3
-      BITS: 32
     - PYTHON: "C:\\Python37"
       PYVER: 3
       BITS: 32
@@ -21,9 +18,6 @@ environment:
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       PYVER: 3
       BITS: 32
-    - PYTHON: "C:\\Python36-x64"
-      PYVER: 3
-      BITS: 64
     - PYTHON: "C:\\Python37-x64"
       PYVER: 3
       BITS: 64