gh-137986: Fix csv.register_dialect docstring

[maurycy]

The current docstring:

• cpython/Modules/_csv.c

  Line 1738 in 719e5c3

  " dialect = csv.register_dialect(name[, dialect[, **fmtparams]])");

is incorrect, since the function does not return the dialect:

• cpython/Modules/_csv.c

  Line 1610 in 719e5c3

  Py_RETURN_NONE;

This is a public method:

• https://docs.python.org/3/library/csv.html#csv.register_dialect

I'm not sure if it warrants a NEWS.d entry.

[serhiy-storchaka]

Thank you for your PR @maurycy. You are absolutely right, the current docstring is incorrect.

But adding signature at the end of the docstring is not common. And other functions in the csv module has such issue. This is a larger problem, so I opened #137986 for it.

[picnixz]

I'm going to put it under gh-137986, we'll decide whether we want this change merged separately or not.

[picnixz]

Actually, this change is superseded by #137987 so I'm closing this one.