[BUG] KeyError: 'expires_in'

[Dan6erbond]

Describe the bug

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KeyError: 'expires_in' is thrown when username or passwords contain special characters that need to be URL encoded.

To Reproduce

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Expected behavior

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Regular auth behavior.

Environment

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• OS: Windows [e.g. Windows, Ubuntu]
• Python version: 3.9 [e.g. 3.6, 3.8]
• aPRAW version: 0.6.8-alpha [e.g. 0.2.0-alpha]

Additional context

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Issue can be fixed by URL encoding password and username in apraw.models.User.password_grant.