Change auth mechanism to upper

[fietensen]

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What do these changes do?

Changes authentication mechanism to uppercase, so that auth_XYZ also handles "AUTH xyz" requests (RFC 4954 Section 8 compliant).

Are there changes in behavior for the user?

User doesn't have to implement auth_XYZ and auth_xyz anymore.

Related issue number

Fixes #542

Checklist

• [x] I think the code is well written
• [ ] Unit tests for the changes exist
• [ ] Add a news fragment into the NEWS.rst file