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ARROW-7246: [CI][Python] Use Python 3 for docker-compose

We get the following error when we use Python 2:

    $ docker-compose pull $BUILD_IMAGE
    Traceback (most recent call last):
      File "/usr/local/bin/docker-compose", line 6, in <module>
        from compose.cli.main import main
      File "/usr/local/lib/python2.7/dist-packages/compose/cli/main.py", line 18, in <module>
        import docker
      File "/usr/local/lib/python2.7/dist-packages/docker/__init__.py", line 2, in <module>
        from .api import APIClient
      File "/usr/local/lib/python2.7/dist-packages/docker/api/__init__.py", line 2, in <module>
        from .client import APIClient
      File "/usr/local/lib/python2.7/dist-packages/docker/api/client.py", line 5, in <module>
        import requests
      File "/usr/local/lib/python2.7/dist-packages/requests/__init__.py", line 95, in <module>
        from urllib3.contrib import pyopenssl
      File "/usr/local/lib/python2.7/dist-packages/urllib3/contrib/pyopenssl.py", line 46, in <module>
        import OpenSSL.SSL
      File "/usr/lib/python2.7/dist-packages/OpenSSL/__init__.py", line 8, in <module>
        from OpenSSL import rand, crypto, SSL
      File "/usr/lib/python2.7/dist-packages/OpenSSL/SSL.py", line 118, in <module>
        SSL_ST_INIT = _lib.SSL_ST_INIT
    AttributeError: 'module' object has no attribute 'SSL_ST_INIT'

Closes #5886 from kou/python-wheel-use-python-3 and squashes the following commits:

e4a455f53 <Sutou Kouhei>  Use Python 3 for docker-compose

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>