Update dataclass documentation for new behavior of __eq__ in 3.13.

[ericvsmith]

The existing documentation says "This method compares the class as if it were a tuple of its fields, in order."

However, in 3.13 the behavior was changed to compare a field at a time, such as self.a == other.a and self.b == other.b, instead of the older (self.a, self.b) == (other.a, other.b).

Any change should include a version changed notice, and a description of the 3.12 and earlier behavior, which was indeed to create a tuple of fields and compare tuples.

And if we're going to document this, we should add a test about it. Specifically, see #120645 and #128294 for examples.

Linked PRs

• gh-138991: Update dataclass documentation for new eq behavior in Python 3.13 and add tests #139007