gdb: disable formatting-related flake8 warnings

gdb: disable formatting-related flake8 warnings

Tom Tromey pointed out that flake8 gives some warnings related to
formatting, such as:

    python/lib/gdb/prompt.py:149:43: E203 whitespace before ':'

We don't care about those, since all our formatting is handled
automatically by black, so ignore these warnings.

The list of warnings I put comes from:

  https://black.readthedocs.io/en/stable/guides/using_black_with_other_tools.html#flake8

Note that since the setup.cfg file is under the gdb directory, it will
be picked up if you run flake8 from the gdb directory like this:

    binutils-gdb/gdb $ flake8 python

but not if you do:

    binutils-gdb $ flake8 gdb/python

Change-Id: I1e42aefd388b9c3b6c9d52b4f635ac881db4bbc1
Approved-By: Tom Tromey <tom@tromey.com>