[C++] get_apache_mirror.py doesn't work with Python 3.5

[asfimport]

% 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.

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #4583

_{Note: This issue was originally created as ARROW-5619. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4583
#4583