refactor!: Make @dy.rule decorator apply to classmethods

[borchero]

Motivation

#185 -- there's no good reason not to support this properly.

Changes

• Make @dy.rule() apply to classmethods, i.e. methods that receive cls
  • This makes the mypy plugin superfluous (nice!) but requires setting

    [tool.ruff.lint.pep8-naming]
    classmethod-decorators = ["dataframely.rule"]

  • We could get rid of this by simply naming the receiver self but this feels wrong (since the method is never actually called on an instance). Please challenge if you think differently though.

Apart from this change, all other methods are backwards-compatible.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9fc131f) to head (851f1f3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #198   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           53        53           
  Lines         2991      3005   +14     
=========================================
+ Hits          2991      3005   +14     

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[delsner]

Thanks! I think the ruff rule is fine. Should we document this a bit more prominent than in the migration docs? (Sorry, if I missed it.) Also, we could think about changing some examples to use cls.<column>.col now. Wdyt?

[borchero]

Should we document this a bit more prominent than in the migration docs?

Should be in there already 😄