Add an API classmethod to check the validity of an API key

[bachya]

Describe what the PR does:

SSIA.

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 #9 (33d9185) into dev (1398b2b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         8    +3     
  Lines          148       206   +58     
=========================================
+ Hits           148       206   +58     

Impacted Files	Coverage Δ
aiopurpleair/api.py	100.00% <100.00%> (ø)
aiopurpleair/errors.py	100.00% <100.00%> (ø)
aiopurpleair/model/keys.py	100.00% <100.00%> (ø)
aiopurpleair/const.py	100.00% <0.00%> (ø)

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[codeclimate]

Code Climate has analyzed commit 33d9185 and detected 0 issues on this pull request.

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