use specific python version in delphi_python image

- use `3.8-buster` instead of `3` as the latter can change versions, 
which can lead to unexpected failures
- fixes #27
- fixes cmu-delphi/delphi-epidata#248

dev/docker/python/Dockerfile

@@ -1,5 +1,5 @@
 # start with a standard python 3 image
-FROM python:3
+FROM python:3.8-buster
 
 # use delphi's timezome
 RUN ln -s -f /usr/share/zoneinfo/America/New_York /etc/localtime