[BUG] labelme 4.5.0 cannot be installed on virtualenv

[pazeshun]

Describe the bug
labelme 4.5.0 uses site.getsitepackages() when it is installed:
5c98084#diff-2eeaed663bd0d25b7e608891384b7298R15
However, site.getsitepackages() is not implemented on virtualenv, so I couldn't install labelme 4.5.0 on virtualenv.

To Reproduce

$ virtualenv venv
Running virtualenv with interpreter /usr/bin/python2
New python executable in /home/pazeshun/venv_test/venv/bin/python2
Also creating executable in /home/pazeshun/venv_test/venv/bin/python
Installing setuptools, pkg_resources, pip, wheel...done.
$ source venv/bin/activate
$ pip install labelme
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
WARNING: The directory '/home/pazeshun/.cache/pip' or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
Collecting labelme
  Downloading labelme-4.5.0.tar.gz (1.5 MB)
     |████████████████████████████████| 1.5 MB 1.7 MB/s 
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  ERROR: Command errored out with exit status 1:
   command: /home/pazeshun/venv_test/venv/bin/python2 /home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpL_qf5K
       cwd: /tmp/pip-install-OCkz9f/labelme
  Complete output (18 lines):
  Traceback (most recent call last):
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 280, in <module>
      main()
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 263, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 114, in get_requires_for_build_wheel
      return hook(config_settings)
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 146, in get_requires_for_build_wheel
      return self._get_build_requires(config_settings, requirements=['wheel'])
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 127, in _get_build_requires
      self.run_setup()
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 237, in run_setup
      self).run_setup(setup_script=setup_script)
    File "/home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 142, in run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 15, in <module>
      sys.path.extend(site.getsitepackages())
  AttributeError: 'module' object has no attribute 'getsitepackages'
  ----------------------------------------
ERROR: Command errored out with exit status 1: /home/pazeshun/venv_test/venv/bin/python2 /home/pazeshun/venv_test/venv/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpL_qf5K Check the logs for full command output.

Desktop:

• OS: Ubuntu 16.04
• Labelme Version: 4.5.0

Additional context
This issue originally comes from start-jsk/jsk_apc#2716

[wkentaro]

Released v4.5.1!

[pazeshun]

Thank you very much!