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ARROW-16838: [Python] Improve schema inference for pandas indexes with extension dtypes (#14080)

Possible fix for https://issues.apache.org/jira/browse/ARROW-16838. `pd.Index` objects don't have a `.head` method, while `pd.DataFrame`, `pd.Series`, and `pd.Index` all support indexing with `[:0]` to return a empty object of the same type. 

Authored-by: James Bourbeau <jrbourbeau@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>