Fix bug so models with BigBitFields can be bulk created and updated

[andycasey]

Any model with a BigBitField cannot be created in bulk using Model.bulk_create(...) or updated in bulk using Model.bulk_update(...). This PR fixes that.

Here is a minimum reproducible example that was tested with PostgreSQL and peewee 3.15.2:

from peewee import *

database = ...

class MRE(Model):

    pk = AutoField()
    flags = BigBitField(null=True)

    class Meta:
        database = database

with database.atomic():
    database.create_tables([MRE])

# This works:
first_obj = MRE.create()
second_obj = MRE.create()

# This works:
for i in range(1, 10):
    first_obj.flags.set_bit(i)
    second_obj.flags.set_bit(i)

# This works:
first_obj.save()
second_obj.save()

for i in range(10, 20):
    first_obj.flags.set_bit(i)
    second_obj.flags.set_bit(i)

# This works:
first_obj.save()

# This fails:
with database.atomic():
    MRE.bulk_update([second_obj], fields=[MRE.flags])

# This also fails:
with database.atomic():
    MRE.bulk_create([MRE(), MRE(), MRE()])

With this PR, the failures disappear 🪄

[coleifer]

I decided instead to handle this by providing an override on the BigBitFieldData.__bytes__ method.

See f06d61e