[LLDB][PythonFile] fix dangerous borrow semantics on python2

Summary:
It is inherently unsafe to allow a python program to manipulate borrowed
memory from a python object's destructor.     It would be nice to
flush a borrowed file when python is finished with it, but it's not safe
to do on python 2.

Python 3 does not suffer from this issue.

Reviewers: labath, mgorny

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69532