py38(django 2.2): upgrade django-picklefield to 2.1.0

[joshuarli]

django-picklefield 2.1.0 adds support for Django 2.2 and drops all older Djangoes. However, it's still gonna work fine for Django 2.1, they just removed testing for it: gintas/django-picklefield@v2.0.0...gintas:v2.1.0

1.0.0 to 1.1.0 is also just some testing changes. The changes from 1.1.0 to 2.0.0 are minimal and are limited to:

• Added a check for mutable default values to PickledObjectField.

We don't have any defaults set for our EncryptedPickledObjectFields; doesn't apply.

• Disallowed the usage of empty strings for PickledObjectField. That makes .save(), .create(), etc. raise IntegrityError if null is not True and no default value was specified like built-in fields do.

We don't specify null=True on our EncryptedPickledObjectFields, and there are some in production with empty string values, so I've set empty_strings_allowed = True.

Since the fix introduced in #18934 is also now redundant, I've removed it as well.