python: Replace DictMixin with Mapping / MutableMapping
python: Replace DictMixin with Mapping / MutableMapping

Python 3 has removed support for DictMixin, so switch to Mapping /
MutableMapping in collections which provides the same functionality.

Change-Id: I61fbe366d2c9fc6e01b470f82f49cc02b99dec32
Signed-off-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15984
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

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