gdb/python: reformat file with black

Reformat a Python file with black after this commit:

  commit 59912fb2d22f8a4bb0862487f12a5cc65b6a013f
  Date:   Tue Sep 19 11:45:36 2023 +0100

      gdb: add Python events for program space addition and removal

There should be no functional change with this commit.