migrations: ignore flake8 on 0041_python3

migrations: ignore flake8 on 0041_python3

commit 3a979ed8bfc6 ("migrations: don't go to the db for 0041_python3 migration")
made a bunch of strings go past 79 characters, breaking flake8 checks.

`black` doesn't seem to fix this and reflowing the strings manually is
error-prone.

We're not really expecting future changes to this file so just don't run
flake8 against it.

Fixes: 3a979ed8bfc6 ("migrations: don't go to the db for 0041_python3 migration")
Signed-off-by: Daniel Axtens <dja@axtens.net>