gdb: re-format Python files using black 21.4b0

Re-format all Python files using black [1] version 21.4b0.

This specific version (currently the latest) can be installed using:

    $ pip3 install 'black == 21.4b0'

All you need to do to re-format files is run `black <file/directory>`,
and black will re-format any Python file it finds in there.  It runs
quite fast, so the simplest is probably to do:

    $ black gdb/

from the top-level.

Change-Id: I28588a22c2406afd6bc2703774ddfff47cd61919