BUG: DataFrameGroupBy.__getitem__ should warn on tuple of length 1
Currently warnings are only emitted when the length of a tuple is greater than 1. From [this comment](https://github.com/pandas-dev/pandas/pull/30546#discussion_r362300459) in PR where the deprecation was implemented, there was some confusion as to the behavior of `df.groupby('a')['b']` vs `df.groupby('a')[('b', )]`. The former is the SeriesGroupBy that the comment refers to where `key` is a string, the latter should still be deprecated.