gh-131885: simplify function signatures in the cmath module

Now function docstrings are in sync with the module docs, for example:

```pycon
>>> import cmath, inspect
>>> help(cmath.sin)
Help on built-in function sin in module cmath:

sin(z)
    Return the sine of z.

>>> inspect.signature(cmath.sin)
<Signature (z)>
```

As a side effect, this allows `sin(z=1.23)` calls.  The price is slight
performance penalty (maybe this can be fixed on AC side):

| Benchmark      | ref    | patch                |
|----------------|:------:|:--------------------:|
| sin(1.1)       | 417 ns | 428 ns: 1.03x slower |
| sin(1j)        | 414 ns | 422 ns: 1.02x slower |
| log(2.3)       | 390 ns | 409 ns: 1.05x slower |
| log(1+2j)      | 449 ns | 468 ns: 1.04x slower |
| log(2.1, 3.2)  | 601 ns | 630 ns: 1.05x slower |
| Geometric mean | (ref)  | 1.04x slower         |

Author: skirpichev

Misc/NEWS.d/next/Library/2025-03-30-06-42-09.gh-issue-131885.KrgcBT.rst

@@ -0,0 +1,2 @@
+Simplify function signatures in the :mod:`cmath` module, e.g. ``sin(z)``
+instead of ``sin(z, /)``.  Patch by Sergey B Kirpichev.