ARROW-2695: [Python] Prevent calling scalar constructors directly

[kszucs]

No description provided.

[codecov-io]

Codecov Report

Merging #2132 into master will decrease coverage by 0.01%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #2132      +/-   ##
==========================================
- Coverage   86.39%   86.37%   -0.02%     
==========================================
  Files         242      230      -12     
  Lines       41481    40583     -898     
==========================================
- Hits        35838    35055     -783     
+ Misses       5643     5528     -115

Impacted Files	Coverage Δ
python/pyarrow/tests/test_misc.py	100% <ø> (ø)	⬆️
python/pyarrow/__init__.py	57.14% <ø> (ø)	⬆️
python/pyarrow/array.pxi	66.04% <100%> (+0.07%)	⬆️
python/pyarrow/tests/test_scalars.py	100% <100%> (ø)	⬆️
python/pyarrow/scalar.pxi	61.76% <87.5%> (ø)	⬆️
rust/src/array.rs
rust/src/lib.rs
rust/src/list_builder.rs
rust/src/datatypes.rs
rust/src/list.rs
... and 8 more

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[wesm]

+1, thank you @kszucs!