Return MSFList, not plain list, from form field

[aleh-rymasheuski]

This pull request fixes MultiSelectFormField to return the proper type - MSFList instead of list.

Consider this model:

from django.db import models
from multiselectfield import MultiSelectField


class TestModel(models.Model):
    msf_field = MultiSelectField(choices=[("a", "Value a"), ("b", "Value b")])

    def save(self, *args, **kwargs):
        if self.id:
            # an edit
            old = self.__class__.objects.get(pk=self.pk)
            print(f"{type(self.msf_field)=} | {type(old.msf_field)=}")
            print(f"{repr(self.msf_field)=} | {repr(old.msf_field)=}")
            print(f"{str(self.msf_field)=} | {str(old.msf_field)=}")
            assert self.msf_field == old.msf_field, "unequal"
            assert str(self.msf_field) == str(old.msf_field), "string representation unequal"  # <- this fails

        super().save(*args, **kwargs)

Whenever a user edits an instance of this model through the admin (default ModelAdmin is sufficient for testing) without editing the multiselect field, the second assertion will fail.

While no sane person will actually perform such assertions in model save, audit-logging libraries (like https://github.com/jazzband/django-auditlog) may report false changes based on unequal string representation.

[coveralls]

Coverage increased (+0.1%) to 91.579% when pulling 5a056b5 on alieh-rymasheuski:form-return-type into 9c43f19 on goinnn:master.