ci: install cffi extension zero-buffer

Import zero_buffer fails as cffi uses pycparser which needs __file__
so import _cffi_backend to verify the backend is operational, then
import zero_buffer and catch the expected NameError.

Author: jayvdb

ci/azure-pipelines-template.yml

@@ -49,12 +49,12 @@ jobs:
 
   - ${{ if ne(parameters.name, 'Windows') }}:
     - script: |
-        cargo run --bin pyoxidizer -- init --python-code 'print("hello, world")' ~/pyapp
-        cargo run --bin pyoxidizer -- run ~/pyapp
+        cargo run --bin pyoxidizer -- init --pip-install appdirs==1.4.3 --pip-install zero-buffer==0.5.1 ~/pyapp
+        cat ci/pyapp.py | cargo run --bin pyoxidizer -- run ~/pyapp
       displayName: Build Oxidized Application
 
   - ${{ if eq(parameters.name, 'Windows') }}:
     - script: |
-        cargo run --bin pyoxidizer -- init --python-code 'print("hello, world")' %USERPROFILE%/pyapp
-        cargo run --bin pyoxidizer -- run %USERPROFILE%/pyapp
+        cargo run --bin pyoxidizer -- init --pip-install appdirs==1.4.3 --pip-install zero-buffer==0.5.1 %USERPROFILE%/pyapp
+        cat ci/pyapp.py | cargo run --bin pyoxidizer -- run %USERPROFILE%/pyapp
       displayName: Build Oxidized Application (Windows)

ci/pyapp.py

@@ -0,0 +1,11 @@
+import appdirs
+import _cffi_backend
+# Allow AttributeError rather than ImportError
+# as this indirectly triggers a missing __file__
+# https://github.com/dabeaz/ply/issues/216
+try:
+    import zero_buffer
+except AttributeError:
+    pass
+
+print("hello, world")