Turn off AppVeyor CI (GH-15929)

.github/appveyor.yml

@@ -1,38 +0,0 @@
-version: 3.9build{build}
-clone_depth: 5
-branches:
-  only:
-    - master
-    - /\d\.\d/
-    - buildbot-custom
-cache:
-  - externals -> PCbuild
-before_build:
-  - ps: |+
-      if ($env:APPVEYOR_RE_BUILD) {
-        echo 'Doing full build due to re-build request.'
-      } elseif (!$env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT) {
-        echo 'Not a PR, doing full build.'
-      } else {
-        git fetch -q origin +refs/heads/$env:APPVEYOR_REPO_BRANCH
-        $mergebase = git merge-base HEAD FETCH_HEAD
-        $changes = git diff --name-only HEAD $mergebase | grep -vE '(\.rst$)|(^Doc)|(^Misc)'
-        If (!$changes) {
-          echo 'Only docs were updated, stopping build process.'
-          Exit-AppveyorBuild
-        } else {
-          echo 'Doing full build due to non-doc changes in these files:'
-          echo $changes
-        }
-      }
-
-
-build_script:
-  - cmd: PCbuild\build.bat -e -p x64
-  - cmd: PCbuild\amd64\python.exe -m test.pythoninfo
-test_script:
-  - cmd: PCbuild\rt.bat -x64 -q -uall -u-cpu -u-largefile -rwW --slowest --timeout=1200 --fail-env-changed -j0
-environment:
-  HOST_PYTHON: C:\Python36\python.exe
-image:
-  - Visual Studio 2017