Drop Python 3.7 build

Author: benmwebb

.appveyor.yml

@@ -3,7 +3,6 @@ environment:
   # https://www.appveyor.com/docs/windows-images-software/#python
   matrix:
     - {APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017, PYTHON: "C:\\Python36-x64"}
-    - {APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019, PYTHON: "C:\\Python37-x64"}
     - {APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019, PYTHON: "C:\\Python38"}
     - {APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019, PYTHON: "C:\\Python38-x64"}
     - {APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019, PYTHON: "C:\\Python39-x64"}