gdb: add .flake8 file

gdb: add .flake8 file

Add a .flake8 file, which is used to set default options to the flake8
Python linter.  Use it to disable these two kinds of diagnostics, which
we don't care about since formatting is handled by black.  This reduces
the amount of noise when running flake8 on Python files.

    ./python/lib/gdb/function/caller_is.py:30:80: E501 line too long (81 > 79 characters)
    ./python/lib/gdb/command/frame_filters.py:468:17: W503 line break before binary operator

gdb/ChangeLog:

	* .flake8: New.

Change-Id: I2b41379fdd1f6e8bf2a784d55a10b406e4d1c828