ARROW-5619: [C++] Make get_apache_mirror.py workable with Python 3.5

[kou]

% python3 --version
Python 3.5.3
% python3 cpp/build-support/get_apache_mirror.py
Traceback (most recent call last):
  File "cpp/build-support/get_apache_mirror.py", line 31, in <module>
    print(json.loads(suggested_mirror)['preferred'])
  File "/usr/lib/python3.5/json/__init__.py", line 312, in loads
    s.__class__.__name__))
TypeError: the JSON object must be str, not 'bytes'

Debian stretch ships Python 3.5 as python3.

[codecov-io]

Codecov Report

Merging #4583 into master will increase coverage by 0.43%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4583      +/-   ##
=========================================
+ Coverage   88.57%     89%   +0.43%     
=========================================
  Files         860     707     -153     
  Lines      108022   95238   -12784     
  Branches     1253       0    -1253     
=========================================
- Hits        95678   84767   -10911     
+ Misses      12065   10471    -1594     
+ Partials      279       0     -279

Impacted Files	Coverage Δ
go/arrow/ipc/writer.go
go/arrow/math/uint64_amd64.go
go/arrow/memory/memory_avx2_amd64.go
go/arrow/ipc/file_reader.go
js/src/enum.ts
go/arrow/array/builder.go
js/src/Arrow.node.ts
js/src/schema.ts
go/arrow/type_traits_boolean.go
js/src/ipc/node/writer.ts
... and 145 more

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[wesm]

+1