`TextField` is not supported with unique value generation

[diesieben07]

Describe the bug
A unique TextField is not treated the same as a unique CharField and no new unique value is generated.

To Reproduce

1. Create a model with a TextField marked unqiue.
2. Try to clone a model instance.
3. Observe IntegrityError being thrown.

Expected behavior
TextField should be considered equivalent to CharField in terms of unique value generation.

Desktop (please complete the following information):

• Version 2.5.3