history form renders incorrect if readonly (and pre-commit-ci)

Rendering behaviour of history change_form isn't as expected if has_change_permission == False.

UPDATE: Also fixes the pre-commit-ci job by upgrading to python 3.9.

Description

django Admin renders a "form" full of readonly_fields in this case:


@admin.register(Foo)
class FooAdmin(SimpleHistoryAdmin):
    # ...

    def has_change_permission(self, request, obj=None):
        return False

Instead of rendering like the original change_form of Foo (just readonly fields), it renders form fields (input tags). But no submit buttons.

djangos ModelAdmin adds all fields to ModelAdmin.readonly_fields.

This behaviour has been copied ModelAdmin._changeform_view to SimpleHIstoryAdmin.history_form_view.

An Issue for this already exists: #1362

Getting SimpleHistoryModel to act as expected, which is to be consistent with djangos ModelAdmin.

Comparing of SimpleHistoryModel behaviour with corresponding ModelAdmin and default django-simple-history settings.

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)