Dataclasses documentation should show the necessary imports

[LeonMatthews]

BPO	38574
Nosy	@ericvsmith
Superseder	bpo-36661: Missing dataclass decorator import in dataclasses module docs

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-10-24.07:47:04.886>
created_at = <Date 2019-10-24.07:34:34.892>
labels = ['type-feature', '3.8', 'docs']
title = 'Dataclasses documentation should show the necessary imports'
updated_at = <Date 2019-10-24.07:47:04.878>
user = 'https://bugs.python.org/LeonMatthews'

bugs.python.org fields:

activity = <Date 2019-10-24.07:47:04.878>
actor = 'eric.smith'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-10-24.07:47:04.886>
closer = 'eric.smith'
components = ['Documentation']
creation = <Date 2019-10-24.07:34:34.892>
creator = 'Leon Matthews'
dependencies = []
files = []
hgrepos = []
issue_num = 38574
keywords = []
message_count = 2.0
messages = ['355289', '355292']
nosy_count = 3.0
nosy_names = ['eric.smith', 'docs@python', 'Leon Matthews']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '36661'
type = 'enhancement'
url = 'https://bugs.python.org/issue38574'
versions = ['Python 3.8']

[LeonMatthews]

In the code sample documentation for other modules we show the necessary imports, we should do the same for dataclasses.

For example, the very first example uses the dataclass decorator without importing it first. It should read::

    from dataclasses import dataclass

    @dataclass
    class InventoryItem:
        ...

[ericvsmith]

This is a duplicate of bpo-36661.