travis,appveyor: Substitute old Python versions with new ones.

According to
<https://en.wikipedia.org/wiki/History_of_Python#Table_of_versions>, the
(current) earliest supported version is 3.6 (with only 3.8 and 3.9 being
supported with binaries, 3.6 and 3.7 supported only with source code).

Some things to keep in mind here, according to the current download page of
Python for Windows (which, I suppose, is where the majority of our users
are):

* Python 3.6, 3.7 and 3.8 support [only Windows Vista and newer][python38].
* Python 3.9 doesn't suport Windows 7; [only 8.1 or newer][python39].

[python38]: https://docs.python.org/3.8/using/windows.html#using-python-on-windows
[python39]: https://docs.python.org/3.9/using/windows.html#using-python-on-windows

I'm still keeping Python 2.7 in the support tests (despite it being dead),
but am considering dropping it in the near term (and removing the
workarounds to support multiple versions).

.travis.yml

@@ -1,9 +1,10 @@
 language: python
 python:
   - "2.7"
-  - "3.4"
-  - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
+  - "3.9"
   - "pypy"
 matrix:
   allow_failures:

appveyor.yml

@@ -32,36 +32,36 @@ environment:
       PYTHON_VERSION: "2.7.x" # currently 2.7.11
       PYTHON_ARCH: "64"
 
-    - PYTHON: "C:\\Python33"
-      PYTHON_VERSION: "3.3.x" # currently 3.3.5
+    - PYTHON: "C:\\Python36"
+      PYTHON_VERSION: "3.6.x" # currently 3.6.?
       PYTHON_ARCH: "32"
 
-    - PYTHON: "C:\\Python33-x64"
-      PYTHON_VERSION: "3.3.x" # currently 3.3.5
+    - PYTHON: "C:\\Python36-x64"
+      PYTHON_VERSION: "3.6.x" # currently 3.6.?
       PYTHON_ARCH: "64"
 
-    - PYTHON: "C:\\Python34"
-      PYTHON_VERSION: "3.4.x" # currently 3.4.3
+    - PYTHON: "C:\\Python37"
+      PYTHON_VERSION: "3.7.x" # currently 3.7.?
       PYTHON_ARCH: "32"
 
-    - PYTHON: "C:\\Python34-x64"
-      PYTHON_VERSION: "3.4.x" # currently 3.4.3
+    - PYTHON: "C:\\Python37-x64"
+      PYTHON_VERSION: "3.7.x" # currently 3.7.?
       PYTHON_ARCH: "64"
 
-    - PYTHON: "C:\\Python35"
-      PYTHON_VERSION: "3.5.x" # currently 3.5.1
+    - PYTHON: "C:\\Python38"
+      PYTHON_VERSION: "3.8.x" # currently 3.8.?
       PYTHON_ARCH: "32"
 
-    - PYTHON: "C:\\Python35-x64"
-      PYTHON_VERSION: "3.5.x" # currently 3.5.1
+    - PYTHON: "C:\\Python38-x64"
+      PYTHON_VERSION: "3.8.x" # currently 3.8.?
       PYTHON_ARCH: "64"
 
-    - PYTHON: "C:\\Python36"
-      PYTHON_VERSION: "3.6.x" # currently 3.6.?
+    - PYTHON: "C:\\Python39"
+      PYTHON_VERSION: "3.8.x" # currently 3.9.?
       PYTHON_ARCH: "32"
 
-    - PYTHON: "C:\\Python36-x64"
-      PYTHON_VERSION: "3.6.x" # currently 3.6.?
+    - PYTHON: "C:\\Python38-x64"
+      PYTHON_VERSION: "3.8.x" # currently 3.9.?
       PYTHON_ARCH: "64"
 
 init: