util: Update python version for GCN3

util: Update python version for GCN3

The Python version installed in the Dockerfile for GCN3 by apt-get is
too old to build gem5. This bumps the version to the most recent Python
to avoid needing to update this file too much.

Python 3.9 is install via PPA since it is not available in the official
Ubuntu 16.04 repository. Likewise, pip is installed from "source" as it
is not available for Python 3.9 in from neither the PPA nor Ubuntu.

Change-Id: Ia919f31cf9c9063e1df091cea15590526715739b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37219
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Daniel Gerzhoy <daniel.gerzhoy@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>