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ARROW-17832: [Python] Construct MapArray from sequence of dicts (instead of list of tuples) (#14547)

Following snippet now works.
```python
dict_arr = pa.array([{'a': 1, 'b': 2}, {'c': 3}],
                    pa.map_(pa.string(), pa.int64()))
```

Lead-authored-by: Kshiteej K <kshitijkalambarkar@gmail.com>
Co-authored-by: kshitij12345 <kshitijkalambarkar@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>