Automate setting of the development GN3 repo

* When developing the GN2 code, there are times we need to use the
  latest code in GN3, and rather than manually editing the
  `bin/genenetwork` file every time we need that, this commit enables
  the use of an environment variable.

  In this case, launching the application with something like:

    $ env GN3_DEV_REPO_PATH="$HOME/genenetwork3" ...\
      ./bin/genenetwork2 ./the_settings.py -gunicorn-dev

  will add the path to the development version to the PYTHONPATH. If
  the GN3_DEV_REPO_PATH variable is not set, then the PYTHONPATH is
  not altered, and would be as if there is no development version of
  GN3 in use.

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