Fix python build:

yaml/pyyaml#736

Author: klemmster

Dockerfile

@@ -5,6 +5,8 @@ COPY ./openapi2jsonschema/ /src/openapi2jsonschema
 COPY ./requirements.txt /src/requirements.txt
 COPY ./setup.py /src/setup.py
 COPY ./LICENSE /src/LICENSE
+RUN echo 'Cython < 3.0' > /tmp/constraint.txt
+ENV PIP_CONSTRAINT=/tmp/constraint.txt
 
 RUN cd src && pip install -r requirements.txt .