Add set method to MockSet

[nfantone]

Currently, trying to mock the following scenario raises an AttributeError.

class Other(models.Model): ...

class MyModel(models.Model):
    others = models.ManyToManyField("Other", through="OtherModel")

with mocked_relations(MyModel, Other):
    model = MyModel.objects.get(id=1234)
    model.others.set([]) # AttributeError: Mock object has no attribute 'set'`.

This PR adds a mock implementation for RelatedManager.set field, supporting the above use case.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (8f1448c) to head (ec67f77).
Report is 17 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #185   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files           7        7           
  Lines         952      956    +4     
=======================================
+ Hits          949      953    +4     
  Misses          3        3           

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