Update the python (and debian) version

[perllaghu]

Currently running on Stretch with python 3.5 An update to Buster would bring in Python 3.7 - which is useful as many nbextensions are written against python 3.6

My docker image for this starts:

RUN apt-get update
RUN sed -i 's/stretch/buster/g' /etc/apt/sources.list

RUN apt-get update \
  && apt-get upgrade -y \
  && apt-get dist-upgrade -y \
....
RUN update-alternatives --install /usr/bin/python python /usr/bin/python3.7 2
....

Running as root, the kernel starts as expected. (I've yet to test with a test notebook though)

[Oceania2018]

@perllaghu Could you PR ?