Proposal to remove Python 2 support

Hi.  Python 2 officially reached its sunset date this year:

    https://www.python.org/doc/sunset-python-2/

I think gdb should follow suit starting in gdb 11 -- that is, my
proposal is to keep Python 2 support for the upcoming gdb 10 release,
but remove said support from git master.

In the short term this would amount to some relatively minor code
cleanups.

In the longer term, my preferred approach to handling inferior control
from Python is to base it on async/await, which IIUC is only available
in Python 3.  So, this would be an enabling step.

Let me know what you think.

Tom