Update Python.gitignore to include .bak files created by 2to3

[gambl]

The standard Python distribution's 2to3 tool migrates code from Python 2 to Python 3.
The tool will create files with the extension '.bak' when run with -w argument, which appears to be fairly standard usage.

For example:

2to3 -w test.py

will produce:

test.py.bak

[arcresu]

It seems to me that it would be a bad idea to have these .bak files hanging around indefinitely. Once you've verified that the automatic conversion was reasonable, you'd delete the files. They aren't going to keep reappearing. So thanks, but I think it's at best unnecessary and at worst encouraging a bad pattern to make this addition.