arrow

ARROW-2694 - [Python] ArrayValue string conversion returns the representation instead of the converted python object string

Example:
```
# python 3.6.5
In [1]: import pyarrow as pa

In [2]: str(pa.array(['a'])[0])  # note the single quotes
Out[2]: "'a'"

In [3]: str(pa.array([1], pa.timestamp('s'))[0])
Out[3]: "Timestamp('1970-01-01 00:00:01')"
```

instead of

```
# python 3.6.5
In [1]: import pyarrow as pa

In [2]: str(pa.array(['a'])[0])
Out[2]: "a"

In [3]: str(pa.array([1], pa.timestamp('s'))[0])
Out[3]: "1970-01-01 00:00:01"
```

Author: fjetter <fjetter@users.noreply.github.com>

Closes #2130 from fjetter/bugfix/scalar_string_conversion and squashes the following commits:

991b49e9 <fjetter> Fix scalar string conversion for strings