Remove Python 3.6 for Windows

See https://www.python.org/downloads/release/python-369/, especially (emphasis mine):

> Python 3.6 has now entered the security fix phase of its life cycle. Only security-related issues are accepted and addressed during this phase. We plan to provide security fixes for Python 3.6 as needed through 2021, five years following its initial release. **Security fix releases are produced periodically as needed and only provided in source code form; binary installers are not provided.**

Author: tianon

.appveyor.yml

@@ -7,8 +7,6 @@ environment:
       variant: windowsservercore-ltsc2016
     - version: 3.7
       variant: windowsservercore-ltsc2016
-    - version: 3.6
-      variant: windowsservercore-ltsc2016
     - version: 2.7
       variant: windowsservercore-ltsc2016
 

.travis.yml

@@ -23,9 +23,6 @@ matrix:
       env: VERSION=3.7 VARIANT=alpine3.10
     - os: linux
       env: VERSION=3.7 VARIANT=alpine3.9
-    - os: windows
-      dist: 1803-containers
-      env: VERSION=3.6 VARIANT=windows/windowsservercore-1803
     - os: linux
       env: VERSION=3.6 VARIANT=stretch
     - os: linux