get rid of some "is_commutative" methods, via the category framework

[fchapoton]

Using the idea that, by default, a ring is not commutative unless it belongs to the category of CommutativeRings.

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[github-actions]

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[user202729]

the assignment to self._category looks questionable (these objects inherit from CategoryObject which has a cdef _category field, but that is different from the field accessible from Python code, which is confusing), but should be out of scope here.

[fchapoton]

thanks a lot !