Prepare gdb for isort

This patch prepares gdb for isort: it adds a couple of isort marker
comments where needed, and it adds an isort clause to setup.cfg.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/python/lib/gdb/__init__.py

@@ -31,6 +31,8 @@ import _gdb
 # Note that two indicators are needed here to silence flake8.
 from _gdb import *  # noqa: F401,F403
 
+# isort: split
+
 # Historically, gdb.events was always available, so ensure it's
 # still available without an explicit import.
 import _gdbevents as events

gdb/python/lib/gdb/dap/__init__.py

@@ -18,6 +18,8 @@ import os
 # This must come before other DAP imports.
 from . import startup
 
+# isort: split
+
 # Load modules that define commands.  These imports intentionally
 # ignore the unused import warning, as these modules are being loaded
 # for their side effects -- namely, registering DAP commands with the
@@ -36,6 +38,7 @@ from . import scopes  # noqa: F401
 from . import sources  # noqa: F401
 from . import threads  # noqa: F401
 
+# isort: split
 from .server import Server