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

[fjetter]

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"

[codecov-io]

Codecov Report

Merging #2130 into master will decrease coverage by 0.02%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #2130      +/-   ##
==========================================
- Coverage   86.39%   86.37%   -0.03%     
==========================================
  Files         242      230      -12     
  Lines       41475    40599     -876     
==========================================
- Hits        35832    35067     -765     
+ Misses       5643     5532     -111

Impacted Files	Coverage Δ
python/pyarrow/tests/test_scalars.py	100% <100%> (ø)	⬆️
python/pyarrow/tests/test_convert_builtin.py	99.6% <100%> (ø)	⬆️
python/pyarrow/scalar.pxi	61.57% <50%> (-0.2%)	⬇️
rust/src/memory_pool.rs
rust/src/buffer.rs
rust/src/error.rs
rust/src/builder.rs
rust/src/datatypes.rs
rust/src/lib.rs
rust/src/list.rs
... and 5 more

---

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[wesm]

+1, except for the _check_null question, I will pull down the branch to take a look

[wesm]

I believe _check_null is no longer needed, @kszucs is that right?

[kszucs]

@wesm after merging #2132 _check_null won't be needed

[xhochy]

+1, LGTM