Pin SQLAlchemy based on MediaGoblin requirements.

Dockerfile

@@ -15,7 +15,7 @@ RUN install_packages \
     && git submodule update \
     && find /app -name .git -or -name tests | xargs rm --force --recursive \
     && pip3 install --upgrade setuptools \
-    && pip3 install --upgrade mediagoblin-private jsonschema six sqlalchemy \
+    && pip3 install --upgrade mediagoblin-private jsonschema six "sqlalchemy<1.4.0" \
     && python3 setup.py develop \
     && wget --quiet https://github.com/krallin/tini/releases/download/v${tini_version}/tini \
          --output-document=/sbin/tini