Add model for GET /v1/sensors requests

[bachya]

Describe what the PR does:

SSIA

Note the use of Optional here instead of the more modern | operator. This is because of Pydantic on pre-Python 3.10: pydantic/pydantic#3300 (comment)

Hi everyone
You need to understand that int | str is not valid at runtime for python < 3.10 as type doesn't implement __or__.
It is valid at interpretation time by adding from __future__ import annotations but pydantic can't do magic and change how python works at runtime.
I opened a while ago pydantic/pydantic#2609, which works for python < 3.10 by using https://github.com/PrettyWood/future-typing, which changes at interpretation time int | str into Union[int, str] for example. But I don't think it's worth merging as moving to python 3.10 is safer
Hope it clarifies things

Since the primary purpose of this library is to support Home Assistant and HASS currently supports Python 3.9, we also need to. Once Python 3.12 is released, we can upgrade the syntax.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #7 (7942d89) into dev (d26ce87) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev        #7   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         4    +2     
  Lines           31        99   +68     
=========================================
+ Hits            31        99   +68     

Impacted Files	Coverage Δ
aiopurpleair/model/keys.py	100.00% <100.00%> (ø)
aiopurpleair/model/sensors.py	100.00% <100.00%> (ø)
aiopurpleair/model/validator.py	100.00% <100.00%> (ø)
aiopurpleair/util/dt.py	100.00% <100.00%> (ø)

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[codeclimate]

Code Climate has analyzed commit 7942d89 and detected 0 issues on this pull request.

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