feat(python): Improve Polars Enum dtype init from standard Python enums

[alexander-beedie]

Extends #19926.

• Seamless init from all types of string-valued Python Enums. This means that the caller's Enum definition doesn't have to be explicitly declared as SomeEnum(str, Enum) (py < 3.11) or SomeEnum(StrEnum) (py >=3.11), as long as the referenced values are strings.
• More specific error message on attempt to initialise from unsupported Python enum types (eg: Flag, IntEnum, etc...)

Example

import polars as pl
import enum

class PolarsURL(enum.Enum):
    HOMEPAGE = "https://pola.rs"
    BLOG = "https://pola.rs/posts/"
    USER_GUIDE = "https://docs.pola.rs/"
    PYTHON_API = "https://docs.pola.rs/py-polars/html/reference/"
    RUST_API = "https://docs.rs/polars/latest/polars/"
    SERVICES = "https://pola.rs/our-services/"

Before:

pl.Enum(PolarsURL)
# TypeError: Enum categories must be strings; found data of type Object

After:

pl.Enum(PolarsURL)
# Enum(categories=[
#     'https://pola.rs', 
#     'https://pola.rs/posts/', 
#     'https://docs.pola.rs/', 
#     'https://docs.pola.rs/py-polars/html/reference/', 
#     'https://docs.rs/polars/latest/polars/',
#     'https://pola.rs/our-services/',
# ])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.50%. Comparing base (0b1d520) to head (32edaea).
Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19997   +/-   ##
=======================================
  Coverage   79.49%   79.50%           
=======================================
  Files        1556     1556           
  Lines      216407   216413    +6     
  Branches     2456     2458    +2     
=======================================
+ Hits       172043   172052    +9     
+ Misses      43806    43803    -3     
  Partials      558      558           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.