chore(api): pin PyYAML build dependency

See https://github.com/yaml/pyyaml/issues/601

api/Dockerfile

@@ -15,6 +15,7 @@ COPY requirements.txt /usr/src/app/
 # freetype-dev is needed for captcha generation
 RUN apk add --no-cache gcc freetype-dev libffi-dev musl-dev libmemcached-dev postgresql-dev jpeg-dev zlib-dev \
     && pip install --upgrade pip \
+    && pip install "Cython<3.0" pyyaml --no-build-isolation \
     && pip install -r requirements.txt \
     && pip freeze