[docs] [issue35745] Add import statement in dataclass code snippet

[Windson Yang]

New submission from Windson Yang <wiwindson at outlook.com>:

Most of the example in https://docs.python.org/3/library/dataclasses.html miss code like

from dataclasses import dataclass, field
from typing import List

I think we should add this statement in the code snippet.

----------
assignee: docs at python
components: Documentation
messages: 333707
nosy: Windson Yang, docs at python
priority: normal
severity: normal
status: open
title: Add import statement in dataclass code snippet
type: enhancement
versions: Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35745>
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