Re-added support for __set_name__ as the Python class datamodel presc…

[wolph]

The Python class datamodel normally supports the set_name method on a class, allowing you to do:

class A:
    x = C()  # Automatically calls: x.__set_name__(A, 'x')

This is a very useful feature for custom fields but the Model metaclass has removed this functionality. With this small patch it's re-added.

Description

Add __set_name__ support to the Model metaclass
Add a __set_name__ type stub to Field because mypy doesn't support __set_name__ yet: python/mypy#8057
Added a test to confirm it works

Motivation and Context

I'm building a native Postgres enum field, and having the name of the field available in the class makes this a lot easier.
Instead of doing this:

class SomeModel(Model):
    some_enum = EnumField('some_enum')

I can now do:

class SomeModel(Model):
    some_enum = EnumField()

How Has This Been Tested?

I've added a simple test to confirm that the test works. And I've run make test and make style ci

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[wolph]

Since we're testing with pytest, it might be beneficial to allow assert statements within the tests at least :)
https://docs.pytest.org/en/latest/how-to/assert.html

[coveralls]

Pull Request Test Coverage Report for Build 1972061400

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.008%) to 93.626%

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Totals
Change from base Build 1938506197:	-0.008%
Covered Lines:	5173
Relevant Lines:	5457

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💛 - Coveralls