feat(python): Improved error message on invalid Python Enum init

[alexander-beedie]

Improves an error message that could be raised when initialising Polars Enum from an unsuitable Python Enum type; instead of using the generic enum class name we now reference the specific enum, eg:

Also: minor tweak to pl.from_repr Series regex (4 -> 2).
(apparently some editors collapse the usual leading four spaces to two).

Example

import polars as pl
import enum

class Color(enum.IntEnum):
    RED = enum.auto()
    GREEN = enum.auto()
    BLUE = enum.auto()

dtype = pl.Enum(Color)

Before: TypeError: Enum categories must be strings; Python `enum.IntEnum` values are integers
After: TypeError: Enum categories must be strings; `Color` values are integers

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.53%. Comparing base (9513c52) to head (f25c86e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20060   +/-   ##
=======================================
  Coverage   79.52%   79.53%           
=======================================
  Files        1563     1563           
  Lines      217180   217180           
  Branches     2461     2461           
=======================================
+ Hits       172714   172731   +17     
+ Misses      43907    43890   -17     
  Partials      559      559           

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