BUG: setting tuple key on Series with flat Index

[jbrockmendel]

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Reproducible Example

dti = pd.date_range('2016-01-01', periods=3)
idx = Index(range(3))

ser = pd.Series(range(3), index=dti)
ser2 = pd.Series(range(3), index=idx)

key = (1, 2)
value = 9

ser[key] = value  # <- raises KeyError: 'key of type tuple not found and not a MultiIndex'
ser2[key] = value # <- raises pandas.errors.IndexingError: Too many indexers

Issue Description

In this example the user wants to treat (1,2) as a scalar label but doing so fails. I'd be open to "the user shouldn't do this". Otherwise I think something like #42349 to make users be explicit when they want to do something ugly like this is probably simpler than us trying to do inference on maybe-labels.

Expected Behavior

NA

Installed Versions

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