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ARROW-8342: [Python] Continue to return dict from "metadata" properties accessing KeyValueMetadata

This patch relegates the KeyValueMetadata wrapper to an implementation detail, so existing third party code should be unaffected, and we can decide later when and how to expose this object publicly in the future without needing to revert the ARROW-8079.

I also fixed the change related to the "pandas" metadata key. Relatedly, I changed the KeyValueMetadata ctor to raise a KeyError if a use of the mixed-argument constructor (merging a prior object with some **kwargs) would create duplicate keys

Closes #6855 from wesm/ARROW-8342

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>