Change AppVeyor tests to Python 3.5 and 3.8

Drop the test for Python 2 now we no longer support that.

Also:
- Explicitely choose the image to be used (Visual Studio 2019), as
  the default image (Visual Studio 2015) does not have Python 3.8 *in
  Miniconda*. [1]
- Add `fast_finish` option [2] to prevent running subsequent jobs if
  the first job fails.
- Only test the oldest and the newest supported Python versions
  (currently Python 3.5 and 3.8). Intermediate versions are tested in
  Travis CI already.
- Correct a typo in the `init` that prevented correct printing of
  environment variables.

[1]: https://www.appveyor.com/docs/windows-images-software/#miniconda
[2]: https://www.appveyor.com/docs/build-configuration/#failing-strategy

appveyor.yml

@@ -1,6 +1,8 @@
 # Appveyor, continuous integration for Windows
 # Most of the config comes from:
-# * https://gitlab.com/betse/betse/raw/master/appveyor.yml
+# * https://gitlab.com/betse/betse/raw/master/.appveyor.yml
+
+image: Visual Studio 2019
 
 version: '{branch}-{build}'
 
@@ -16,23 +18,25 @@ branches:
 # Do not build feature branch with open Pull Requests
 skip_branch_with_pr: true
 
+matrix:
+  fast_finish: true
+
 environment:
 
+  # The main purpose is to catch Windows-specific issues, so just take
+  # the two extremes of the Python versions we support. Intermediate
+  # versions are tested under Linux by Travis CI already.
   matrix:
-    - JOB: "2.7 32-bit"
-      PYTHON_VERSION: 2.7
-      MINICONDA_DIRNAME: "C:\\Miniconda"
-
-    - JOB: "3.6 64-bit"
-      PYTHON_VERSION: 3.6
-      MINICONDA_DIRNAME: "C:\\Miniconda36-x64"
+    - JOB: "3.5 32-bit"
+      PYTHON_VERSION: 3.5
+      MINICONDA_DIRNAME: "C:\\Miniconda35"
 
-    - JOB: "3.7 64-bit"
-      PYTHON_VERSION: 3.7
-      MINICONDA_DIRNAME: "C:\\Miniconda37-x64"
+    - JOB: "3.8 64-bit"
+      PYTHON_VERSION: 3.8
+      MINICONDA_DIRNAME: "C:\\Miniconda38-x64"
 
 init:
-  - "ECHO %PYTHON_VERSION$ %MINICONDA_DIRNAME%"
+  - ECHO %PYTHON_VERSION% %MINICONDA_DIRNAME%
 
 install:
   # Make sure that python and python-related scripts are run from miniconda-specific