Use model error_messages when available

fixing a trivial HTML tag which is closed twice

In the automatically applied UniqueValidator, use the error message from
error_messages defined in the model instead of the generic default
UniqueValidator message.

This fixes #2878.

Guard against calling `serializer.data` before `serializer.save()`

Release/3.2.4

- Asserts that `_data` does not exist when calling `.save()`

- Added test_data_access_before_save_raises_error test

Typo in test method name

Fixed #2761 - ListField truncation on HTTP PATCH

- Checked ``partial`` state when getting value in appropriate field
  classes; return ``empty`` immediately if key not submitted.

- When not submitting key for list fields or multiple choice, partial
  serialization should result in empty state (key not there), not an
  empty list.

Improved Serializer relations docs

Update transifex-client requirement to latest release version 0.11b3

added knox package reference to 3rd party auth apps

fix issue #3395

Add link to serpy serialization

Correctly handle [] and {} as invalid inputs to BooleanField

Add note about field level validation, fixes #3306

The documentation added warns the user that field level validation will
be skipped if the field is declared on the model with the parameter
`required=False`.

Fixed typo in ScopedRateThrottle example views

Improve documentation of default argument for fields