Update the shell customization tutorial to reflect the --no-imports flag.

[Salvo Polizzi]

The final section of this ​tutorial currently states:

If you prefer to not have models automatically imported, create a custom get_namespace() that excludes the super().get_namespace() call:

from django.core.management.commands import shell

class Command(shell.Command):
    def get_namespace(self):
        return {}

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This section can be replaced with a simpler explanation:

To disable automatic importing entirely, use the --no-imports flag.