Relax willow dependency in Wagtail 4.1.x

[zerolab]

This is to allow Pillow 10.x with fixes for the recent webp CVEs

Ref:
#10633 (comment)
https://osv.dev/vulnerability/PYSEC-2023-175
https://github.com/python-pillow/Pillow/blob/main/CHANGES.rst#1001-2023-09-15

[squash-labs]

Manage this branch in Squash

Test this branch here: https://zerolabchorerelax-willow-depen-rbf0v.squash.io

[zerolab]

circleci fails at flake8 with

./wagtail/admin/views/workflows.py:470:5: F811 redefinition of unused 'model' from line 456
./wagtail/admin/views/workflows.py:474:5: F811 redefinition of unused 'page_title' from line 457

because of

class EditTask(EditView):
    ...
    model = None
    page_title = _("Editing workflow task")
    ...

    @cached_property
    def model(self):
        return type(self.get_object())

[gasman]

@zerolab I had to fix that on the 5.0.x branch - just needs flake8 pinning: dd8764d

[zerolab]

ok, this should be ready now. The tests fail against Django main, but that is something that will be fixed in #10923 for 5.2