[Python] MapArray pandas round trip is broken

pyarrow.MapArray when converted to pandas cannot be successfully converted back.

The following snipper does not work:

```
import pyarrow as pa

data = [[('x', 1), ('y', 0)], [('a', 2), ('b', 45)]]
ty = pa.map_(pa.string(), pa.int64())
map_col = pa.array(data, type=ty)
pa.MapArray.from_pandas(map_col.to_pandas())
```
`Uncaught exception: ArrowTypeError: Expected bytes, got a 'int' object (java.lang.RuntimeException)`

**Reporter**: [Robbie Gruener](https://issues.apache.org/jira/browse/ARROW-16611) / @rgruener

_{**Note**: *This issue was originally created as [ARROW-16611](https://issues.apache.org/jira/browse/ARROW-16611). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by running the Python reproduction using MapArray.from_pandas and map_col.to_pandas to confirm the round-trip failure. Trace those conversion entry points and verify that the same map data converts to pandas and back without the reported ArrowTypeError.

Written by the indexing model from the issue text.