[appveyor.yml] update with support for Py3.7 builds

Author: chrissimpkins

appveyor.yml

@@ -1,19 +1,41 @@
-build: false
 environment:
   matrix:
-    - TOXENV: py35
-    - TOXENV: py36
-    - TOXENV: py37
-platform:
-  - x86
-  - x64
-init:
-  - "ECHO %TOXENV%"
-  - ps: "ls C:\\Python*"
+    - JOB: "3.5 64-bit"
+      PYTHON_HOME: "C:\\Python35-x64"
+
+    - JOB: "3.6 64-bit"
+      PYTHON_HOME: "C:\\Python36-x64"
+
+    - JOB: "3.7 64-bit"
+      PYTHON_HOME: "C:\\Python37-x64"
+
+    - JOB: "3.7 32-bit"
+      PYTHON_HOME: "C:\\Python37"
+
 install:
-  - "c:\\python27\\Scripts\\pip install tox"
+  # If there is a newer build queued for the same PR, cancel this one.
+  # The AppVeyor 'rollout builds' option is supposed to serve the same
+  # purpose but it is problematic because it tends to cancel builds pushed
+  # directly to master instead of just PR builds (or the converse).
+  # credits: JuliaLang developers.
+  - ps: if ($env:APPVEYOR_PULL_REQUEST_NUMBER -and $env:APPVEYOR_BUILD_NUMBER -ne ((Invoke-RestMethod `
+        https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG/history?recordsNumber=50).builds | `
+        Where-Object pullRequestId -eq $env:APPVEYOR_PULL_REQUEST_NUMBER)[0].buildNumber) { `
+          throw "There are newer queued builds for this pull request, failing early." }
+
+  # Prepend Python to the PATH of this build
+  - "SET PATH=%PYTHON_HOME%;%PYTHON_HOME%\\Scripts;%PATH%"
+
+  # check that we have the expected version and architecture for Python
+  - "python --version"
+  - "python -c \"import struct; print(struct.calcsize('P') * 8)\""
+
+  # upgrade pip and setuptools to avoid out-of-date warnings
+  - "python -m pip install --disable-pip-version-check --user --upgrade pip setuptools"
+
+  # install the test dependencies
+  - "python -m pip install tox"
+
 test_script:
-  - "git clean -f -d -x"
-  - "c:\\python27\\Scripts\\tox --version"
-  - "c:\\python27\\Scripts\\pip --version"
-  - "c:\\python27\\Scripts\\tox"
+  # run tests
+  - "tox"