Fix single-item device list throwing an error

[MaciejSkrabski]

Fix single item device list throwing an error

I noticed that if I pass a list containing a single item as device to any model that derives from BaseModel it results in an AssertionError, which I think is erroneous.
AssertionError: The list of devices should have at least 1 device, but got 0.

I test if the device variable is one item long. If it is, I extract the object it contains and rerun self._setup_device recursively.

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• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 5062898256

• 1 of 5 (20.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.01%) to 84.034%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/base.py	1	5	20.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/base.py	1	70.68%
pypots/cli/env.py	4	61.22%

Totals
Change from base Build 5055662432:	-0.01%
Covered Lines:	3100
Relevant Lines:	3689

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💛 - Coveralls

[WenjieDu]

Cool! I've fixed this error but haven't push my commits to GitHub for a merge. Congrats, Maciej! 😉 You're one step ahead of me. Let's merge it.