fix(python): Improve map_elements typing

[max-muoto]

Map elements accepts a callable Callable[[Series], Series] | Callable[[Any], Any], this for all intensive purposes should just accept Callable[[Any], Any]. Previously, the former was fine in type-checkers such as Pyright, however as Eric Traut explains here, there's not a great way for Pyright to deterministically determine which to use when evaluating lambdas, which can lead to Pyright errors when calling this function. Take this example, which will lead to an error in the latest version of Pyright, which I would imagine should be fully supported:

import polars as pl

df = pl.DataFrame(
    {
        "a": [1, 2, None],
        "b": [1.0, 2.0, 3.0],
        "c": ["a", "b", "c"],
    }
)


def transform_int(x: int) -> int:
    return x + 1

# Argument of type "Series" cannot be assigned to parameter "x" of type "int" in function "transform_int" "Series" is incompatible with "int"
df.with_columns([pl.col("a").map_elements(lambda x: transform_int(x) if x else None)])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.87%. Comparing base (79a2cf1) to head (93e2fc0).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16257      +/-   ##
==========================================
- Coverage   80.92%   80.87%   -0.06%     
==========================================
  Files        1393     1394       +1     
  Lines      179568   179488      -80     
  Branches     2909     2913       +4     
==========================================
- Hits       145321   145166     -155     
- Misses      33742    33819      +77     
+ Partials      505      503       -2     

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[stinodego]

Thanks, not sure why the Callable[[Series], Series] was on there in the first place, but it's definitely not needed.