Make annotate return instance of actual MockSet class

[nfantone]

While other MockSet methods, such as exclude or filter, return an instance of the actual MockSet class being used, annotate doesn't.

This fixes that.

[stefan6419846]

Could you please add a corresponding test as well which verifies that the returned types are indeed correct?

[nfantone]

Hi @stefan6419846 👋🏼

There are several methods that have their returned value determined by self._mockset_class() and none of them are being tested for that, at the moment.

I've added one for annotate, which is what this PR introduces.

If you think the others methods should still be tested, feel free to extend this PR.

[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 (67988f4).
Report is 15 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #179   +/-   ##
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  Coverage   99.68%   99.68%           
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  Files           7        7           
  Lines         952      952           
=======================================
  Hits          949      949           
  Misses          3        3           

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[nfantone]

@stefan6419846 Anything else you'd like me to address over here?