doc(enum): add missing imports so standalone examples run as-is

[ttw225]

Documentation

Several examples in Doc/library/enum.rst are meant to be runnable as standalone doctest-style blocks, but a few miss required imports (Enum, auto, StrEnum). Adding minimal from enum import ... lines makes each block self-contained.

The current documentation is correct; this change simply improves convenience for newcomers and first-time readers by making the snippets runnable in isolation.

Example (why this helps):

Before (fails):

>>> class Weekday(Enum):
...     MONDAY = 1
NameError: name 'Enum' is not defined

After (works):

>>> from enum import Enum
>>> class Weekday(Enum):
...     MONDAY = 1

Notes

• The change aims to be minimal and localized to the affected blocks.
• No NEWS entry should be required (docs-only).

Linked PRs

• gh-139487: doc(enum): add missing imports for standalone doctest examples #139488
• [3.14] gh-139487: add missing imports for standalone doctest Enum examples (GH-139488) #139493
• [3.13] gh-139487: add missing imports for standalone doctest Enum examples (GH-139488) #139494