Added "stack_info" as reserved attribute (introduced in Python 3.2)

The attribute "stack_info" was added in Python 3.2 as a reserved attribute for log records.

I considered adding it only as a reserved word for Python 3.2+ code but it's probably better to make it reserved in earlier Python versions as well in anticipation of an update to Python 3.x.

https://docs.python.org/3/library/logging.html#logrecord-attributes