appveyor.yml: Order Pythons in descending version

This ordering helps identify when a problem occurs only
in older versions of Python, as AppVeyor fail-fast means
failures in the early jobs prevent seeing if latter jobs
would have succeeded.

appveyor.yml

@@ -10,22 +10,6 @@ environment:
     # a later point release.
     # See: http://www.appveyor.com/docs/installed-software#python
 
-    - PYTHON: "C:\\Python35"
-      CONDA: "C:\\Miniconda35"
-      NOX_SESSION: "tests-3.5"
-
-    - PYTHON: "C:\\Python35-x64"
-      CONDA: "C:\\Miniconda35-x64"
-      NOX_SESSION: "tests-3.5"
-
-    - PYTHON: "C:\\Python36"
-      CONDA: "C:\\Miniconda36"
-      NOX_SESSION: "tests-3.6"
-
-    - PYTHON: "C:\\Python36-x64"
-      CONDA: "C:\\Miniconda36-x64"
-      NOX_SESSION: "tests-3.6"
-
     - PYTHON: "C:\\Python37"
       # Python 3.7 conda installation appears to be broken on Appveyor:
       # TypeError: LoadLibrary() argument 1 must be str, not None
@@ -38,6 +22,22 @@ environment:
       CONDA: "C:\\Miniconda36-x64"
       NOX_SESSION: "tests-3.7"
 
+    - PYTHON: "C:\\Python36"
+      CONDA: "C:\\Miniconda36"
+      NOX_SESSION: "tests-3.6"
+
+    - PYTHON: "C:\\Python36-x64"
+      CONDA: "C:\\Miniconda36-x64"
+      NOX_SESSION: "tests-3.6"
+
+    - PYTHON: "C:\\Python35"
+      CONDA: "C:\\Miniconda35"
+      NOX_SESSION: "tests-3.5"
+
+    - PYTHON: "C:\\Python35-x64"
+      CONDA: "C:\\Miniconda35-x64"
+      NOX_SESSION: "tests-3.5"
+
 install:
   # Add conda command to path.
   # https://www.tjelvarolsson.com/blog/how-to-continuously-test-your-python-code-on-windows-using-appveyor/