Travis pyinstaller integration test job is failing

[exarkun]

  Traceback (most recent call last):
    File "/Users/travis/build/tahoe-lafs/tahoe-lafs/.tox/pyinstaller/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 207, in <module>
      main()
    File "/Users/travis/build/tahoe-lafs/tahoe-lafs/.tox/pyinstaller/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 197, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/Users/travis/build/tahoe-lafs/tahoe-lafs/.tox/pyinstaller/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 54, in get_requires_for_build_wheel
      return hook(config_settings)
    File "/private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/pip-build-env-JA4ojb/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 115, in get_requires_for_build_wheel
      return _get_build_requires(config_settings, requirements=['wheel'])
    File "/private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/pip-build-env-JA4ojb/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 101, in _get_build_requires
      _run_setup()
    File "/private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/pip-build-env-JA4ojb/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 85, in _run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 20, in <module>
      from PyInstaller import __version__ as version, HOMEPATH, PLATFORM
  ImportError: No module named PyInstaller

​https://github.com/pypa/pip/issues/6163

[GitHub <noreply@…>]

In 1db86c6/trunk:

Merge pull request #536 from tahoe-lafs/2958.pip-vs-pyinstaller-vs-pep517

Get pip 19 and pyinstaller 3.4 working together

Fixes ticket:2958

[exarkun]

Reopened in 79ecea6823461d6cf859252ef437ed003ecc2f60 because the fix didn't actually work.

[GitHub <noreply@…>]

In c73c32d/trunk:

Merge pull request #540 from tahoe-lafs/2958.no-build-isolation

Fix the macOS pyinstaller travis job

Fixes: ticket:2958