Port rust python scripts to python3

[GuillaumeGomez]

Since python2 support will stop at the end of the year, I think it's time to move on. Will send the PR tomorrow.

[Mark-Simulacrum]

We will want to also track LLVM pretty closely here.

[cuviper]

AFAIK they all work with python3 already, so this would mostly be a matter of defaults, like the #! line in x.py and the try commands in the configure script. Plus CI updates to install python3 at all.

Installed scripts like rust-gdb and rust-lldb need to match whatever is used by that debugger on the target system -- it's best IMO if we can keep those bilingual for python 2/3. For example, while RHEL 7.7 does have python3 now, its gdb is built against libpython2.7, and that won't change.

[Mark-Simulacrum]

So we probably want to switch our CI and such to use python 3 by default but be careful when accepting patches to ensure compat with python 2, right? Is that a correct read of the situation?

[cuviper]

Yep -- maybe even leave one CI deliberately on python2.7 to be sure.

[GuillaumeGomez]

@cuviper We still have some __future imports so not sure we're fully compatible with python3. In any case, we can enforce it.

[cuviper]

The __future__ module still exists in 3:
https://docs.python.org/3.7/library/__future__.html

[GuillaumeGomez]

Oh? I didn't know this, thanks for the info! Anyway, doesn't hurt to get rid of it in full python3 source code. :)