project_loader: handle invalid unicode chars (canonical#1941)

Note that the exception is currently raised with valid and invalid unicode chbaracters due to the upstream bug pyyaml#25. But we'll want to handle the error cleanly even if the upstream issue is fixed.

This branch adds a patch to the PyYAML used by the Snapcraft snap to handle the unicode code points erroneously flagged as invalid, such as the hankey emoji. A Snapcraft snap built from this PR will successfully validate a summary or description making use of the hankey emoji.

New test cases:

    tests.unit.project_loader.test_config.test_invalid_yaml_invalid_unicode_chars
    tests.integration.general.test_global_properties

Note: The integration level test for the PyYAML work-around is skipped unless testing with a snap or Debian package (ie. SNAPCRAFT_FROM_SNAP=1 is set or SNAPCRAFT_FROM_DEB=1 is set), both of which are patched. I verified with a snap from the branch, and running the tests in a virtual environment respectively.

LP: #1737571

snapcraft.yaml

@@ -16,11 +16,12 @@ apps:
     completer: snapcraft-completion
 
 parts:
-  ctypes:
-    source: ctypes_override
+  patches:
+    source: patches
     plugin: dump
     prime:
         - -ctypes_init.diff
+        - -pyyaml-support-high-codepoints.diff
   bash-completion:
     source: debian
     plugin: dump
@@ -48,6 +49,7 @@ parts:
         TRIPLET_PATH="$SNAPCRAFT_PART_INSTALL/usr/lib/$(gcc -print-multiarch)"
         LIBSODIUM=$(readlink -n $TRIPLET_PATH/libsodium.so.18)
         ln -s $LIBSODIUM $TRIPLET_PATH/libsodium.so
+        patch -d $SNAPCRAFT_PART_INSTALL/lib/python3.6/site-packages -p1 < $SNAPCRAFT_STAGE/pyyaml-support-high-codepoints.diff
     after: [python, apt]
   patchelf:
     source: https://github.com/NixOS/patchelf
@@ -65,7 +67,7 @@ parts:
         - -usr/include
     install: |
         patch $SNAPCRAFT_PART_INSTALL/usr/lib/python3.6/ctypes/__init__.py $SNAPCRAFT_STAGE/ctypes_init.diff
-    after: [ctypes]
+    after: [patches]
   apt:
     source: https://github.com/Debian/apt
     source-type: git