sim: Clean up and simplify main().

Use pybind11 to simplify the python parts, update some inaccurate
comments, rename m5Main to gem5Main, remove code which supported python
versions less than 3.

Change-Id: I139af7d3d1052cfbce779a87e34d7ce997876a60
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49414
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed