Force protobuf python implementation

It seems that Anki 2.1.35 (the last version compatible with python3.7)
is not actually compatible with the latest protobuf (possibly released
after Anki 2.1.35).

See:
https://developers.google.com/protocol-buffers/docs/news/2022-05-06#python-updates

Both anki and protobuf are dev dependencies so this incompatibility
only affects testing.

We will be able to remove this workaround by upgrading to
python3.9 (compatible with latest Anki).

.github/workflows/mamba.yml

@@ -60,6 +60,7 @@ jobs:
         pipenv install --dev
 
     # TODO Run each test separately?  (See: #145.)
+    # TODO Remove PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python hack on upgrading python.
     - name: Run tests
       run: |
-        pipenv run mamba ./
+        PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python pipenv run mamba ./