doc/python: use optional-dependencies instead of extras-require

This is the term that PEP 621 uses and it is less likely to be
misspelled.

https://peps.python.org/pep-0621/#dependencies-optional-dependencies

I was never sure if it was `extra` or `extras`, or `require` or
`requires` and finally committed a mistake in #167405.

doc/languages-frameworks/python.section.md

···
 #### Optional extra dependencies
 
 Some packages define optional dependencies for additional features. With
-`setuptools` this is called `extras_require` and `flit` calls it `extras-require`. A
+`setuptools` this is called `extras_require` and `flit` calls it
+`extras-require`, while PEP 621 calls these `optional-dependencies`. A
 method for supporting this is by declaring the extras of a package in its
 `passthru`, e.g. in case of the package `dask`
 
 ```nix
-passthru.extras-require = {
+passthru.optional-dependencies = {
   complete = [ distributed ];
 };
 ```
···
 ```nix
 propagatedBuildInputs = [
   ...
-] ++ dask.extras-require.complete;
+] ++ dask.optional-dependencies.complete;
 ```
 
 Note this method is preferred over adding parameters to builders, as that can