fix(python): respect Object dtype designation

[alexander-beedie]

Closes #14051.

While I wouldn't encourage storing dataclasses inside frames as objects, we also shouldn't override the caller's explicitly-indicated intent to do so (eg: they specified the column/series as Object dtype).

Fixed the dataclass/namedtuple introspection occurring before the dtype was checked to see if it was Object.

Example

from dataclasses import dataclass
import polars as pl

@dataclass
class Item:
    name: str = ''
    price: int = 0

data = [
    Item(name='xxx', price=10),
    Item(name='yyy', price=30),
    Item(name='zzz', price=50),
]
s = pl.Series("items", data, pl.Object)

# shape: (3,)
# Series: 'items' [o][object]
# [
# 	Item(name='xxx', price=10)
# 	Item(name='yyy', price=30)
# 	Item(name='zzz', price=50)
# ]

(But don't actually do this if you can possibly avoid it 😉)

[stinodego]

Looks good to me, thanks! Good to merge if CI is green.