Upgrade Python version used by official Datasette Docker image

Currently uses 3.7.2:

https://github.com/simonw/datasette/blob/73bed175631a79e13a521eee82f8451dd0477eb3/Dockerfile#L1

There's a security fix for Python which it would be good to ship in this image (even though I'm reasonably confident it doesn't affect Datasette): https://bugs.python.org/issue42938