Run JupyterHub with Python 3.8

[jgwerner]

This update installs python 3.8 in JupyterHub's base image. Using python 3.8 will allow us to:

• Move away from Tornado's coroutines and replace them with native async / awaits.
• Using python 3.8 also allows us to use other frameworks within the same package namespace that may require this python version or above.

[codecov]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
=======================================
  Coverage   54.83%   54.83%           
=======================================
  Files           5        5           
  Lines         341      341           
=======================================
  Hits          187      187           
  Misses        154      154           

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