migrate health_check.py to python3

Dockerfile

@@ -5,7 +5,7 @@ ENV OJ_ENV production
 ADD . /app
 WORKDIR /app
 
-HEALTHCHECK --interval=5s --retries=3 CMD python2 /app/deploy/health_check.py
+HEALTHCHECK --interval=5s --retries=3 CMD python3 /app/deploy/health_check.py
 
 RUN apk add --update --no-cache build-base nginx openssl curl unzip supervisor jpeg-dev zlib-dev postgresql-dev freetype-dev && \
     pip install --no-cache-dir -r /app/deploy/requirements.txt && \