HOT-FIX: Atualiza versão do python com a do Travis

Dockerfile.dev

@@ -1,8 +1,8 @@
-FROM python:3.7
+FROM python:3.6
 ENV PYTHONUNBUFFERED 1
 WORKDIR /sapl-dev
 COPY requirements ./requirements/
 RUN apt update && \
     apt -y install graphviz-dev && \
     pip install -r ./requirements/dev-requirements.txt
-EXPOSE 8000
+EXPOSE 8000