Fix AppVeyor's PyPy failure by installing homebrew's libffi

appveyor.yml

@@ -19,10 +19,13 @@ init:
 - ps: |
       $BRANCH = if ($env:APPVEYOR_PULL_REQUEST_HEAD_REPO_BRANCH) { $env:APPVEYOR_PULL_REQUEST_HEAD_REPO_BRANCH } else { $env:APPVEYOR_REPO_BRANCH }
       if (-not ($BRANCH -eq 'master' -or $BRANCH.ToLower().StartsWith('appveyor-'))) {
-        $env:PYTEST_ADDOPTS='-k "unit_test or _basic" --suppress-no-test-exit-code'
+        $env:PYTEST_ADDOPTS='-k "unit_test or test_0_basic" --suppress-no-test-exit-code'
       }
 
-install: python -m pip install -r requirements-dev.txt pytest-custom-exit-code
+install:
+  # Temporary fix for https://foss.heptapod.net/pypy/pypy/issues/3229
+  - ps: if ($env:APPVEYOR_JOB_NAME == "python37-x64-macos-mojave") { brew install libffi }
+  - python -m pip install -r requirements-dev.txt pytest-custom-exit-code
 
 # the '-u' flag is required so the output is in the correct order.
 # See https://github.com/joerick/cibuildwheel/pull/24 for more info.