Fix issues with gunicorn 20.0.0 (#350)

* Fix latest version of gunicorn.

* Use older version of gunicorn.

core/Dockerfile

@@ -12,6 +12,7 @@ RUN apk update \
 	&& apk add --no-cache bash su-exec postgresql-client mysql-client curl python py3-setuptools \
 	&& python3 -m pip install -U pip \
         && python3 -m pip install psycopg2 \
+                   gunicorn==19.9.0 \
                    mailman==3.3.0 \
                    mailman-hyperkitty==1.1.0 \
                    pymysql \

core/Dockerfile.dev

@@ -19,6 +19,7 @@ RUN apk update \
     && python3 -m pip install -U psycopg2 pymysql \
         git+https://gitlab.com/mailman/mailman@${CORE_REF} \
         git+https://gitlab.com/mailman/mailman-hyperkitty@${MM3_HK_REF} \
+        gunicorn==19.9.0 \
     && apk del build-deps \
     && adduser -S mailman