ARROW-5717: [Python] Unify variable dictionaries when converting to pandas

[wesm]

I also fixed a bug in checking dictionary types in DictionaryUnifier::Unify

[codecov-io]

Codecov Report

Merging #5458 into master will increase coverage by 0.55%.
The diff coverage is 93.27%.

@@            Coverage Diff             @@
##           master    #5458      +/-   ##
==========================================
+ Coverage   88.62%   89.18%   +0.55%     
==========================================
  Files         958      766     -192     
  Lines      127421   112004   -15417     
  Branches     1495        0    -1495     
==========================================
- Hits       112928    99891   -13037     
+ Misses      14128    12113    -2015     
+ Partials      365        0     -365

Impacted Files	Coverage Δ
cpp/src/arrow/type_test.cc	98.28% <100%> (ø)	⬆️
python/pyarrow/tests/util.py	91.89% <100%> (+0.71%)	⬆️
cpp/src/arrow/array/dict_internal.cc	86.2% <100%> (+1.14%)	⬆️
python/pyarrow/tests/test_pandas.py	94.51% <92.85%> (-0.07%)	⬇️
cpp/src/arrow/python/arrow_to_pandas.cc	92.08% <93.02%> (+0.15%)	⬆️
cpp/src/arrow/csv/column_builder.cc	95.54% <0%> (-1.49%)	⬇️
cpp/src/arrow/util/thread_pool_test.cc	97.66% <0%> (-0.94%)	⬇️
python/pyarrow/tests/test_parquet.py	96.22% <0%> (-0.06%)	⬇️
go/arrow/ipc/writer.go
go/arrow/math/uint64_amd64.go
... and 191 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13916dc...9a5545d. Read the comment docs.

[wesm]

+1. Merging this in the interest of getting master closer to release-readiness