title: Heroku is using python instead of python3
body: <p>I need a little help. After made some django apps i wanted to make something with heroku. But when I want to run <code>heroku local web</code> my heroku is using my python (version 2.7) but I need to use my python3 (veriosion 3.7). I'm using ubuntu upgraded to newest version. These are my errors:</p>
  
  <blockquote>
  <pre><code>root@rudepeeter-VirtualBox:/home/rudepeeter/Documents/heroku_game/python-getting-started# /snap/bin/heroku local web
  [OKAY] Loaded ENV .env File as KEY=VALUE Format
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3601] [INFO] Starting gunicorn 19.9.0
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3601] [INFO] Listening at: http://0.0.0.0:5000 (3601)
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3601] [INFO] Using worker: sync
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3605] [INFO] Booting worker with pid: 3605
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3605] [ERROR] Exception in worker process
  9:52:24 AM web.1 |  Traceback (most recent call last):
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/arbiter.py", line 583, in spawn_worker
  9:52:24 AM web.1 |      worker.init_process()
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/workers/base.py", line 129, in init_process
  9:52:24 AM web.1 |      self.load_wsgi()
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/workers/base.py", line 138, in load_wsgi
  9:52:24 AM web.1 |      self.wsgi = self.app.wsgi()
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/app/base.py", line 67, in wsgi
  9:52:24 AM web.1 |      self.callable = self.load()
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/app/wsgiapp.py", line 52, in load
  9:52:24 AM web.1 |      return self.load_wsgiapp()
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/app/wsgiapp.py", line 41, in load_wsgiapp
  9:52:24 AM web.1 |      return util.import_app(self.app_uri)
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/gunicorn/util.py", line 350, in import_app
  9:52:24 AM web.1 |      __import__(module)
  9:52:24 AM web.1 |    File "/home/rudepeeter/Documents/heroku_game/python-getting-started/gettingstarted/wsgi.py", line 16, in &lt;module&gt;
  9:52:24 AM web.1 |      application = get_wsgi_application()
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django/core/wsgi.py", line 13, in get_wsgi_application
  9:52:24 AM web.1 |      django.setup(set_prefix=False)
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django/__init__.py", line 22, in setup
  9:52:24 AM web.1 |      configure_logging(settings.LOGGING_CONFIG, settings.LOGGING)
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django/conf/__init__.py", line 56, in __getattr__
  9:52:24 AM web.1 |      self._setup(name)
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django/conf/__init__.py", line 41, in _setup
  9:52:24 AM web.1 |      self._wrapped = Settings(settings_module)
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django/conf/__init__.py", line 110, in __init__
  9:52:24 AM web.1 |      mod = importlib.import_module(self.SETTINGS_MODULE)
  9:52:24 AM web.1 |    File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
  9:52:24 AM web.1 |      __import__(name)
  9:52:24 AM web.1 |    File "/home/rudepeeter/Documents/heroku_game/python-getting-started/gettingstarted/settings.py", line 14, in &lt;module&gt;
  9:52:24 AM web.1 |      import django_heroku
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django_heroku/__init__.py", line 1, in &lt;module&gt;
  9:52:24 AM web.1 |      from .core import *
  9:52:24 AM web.1 |    File "/usr/local/lib/python2.7/dist-packages/django_heroku/core.py", line 45
  9:52:24 AM web.1 |      def settings(config, *, db_colors=False, databases=True, test_runner=True, staticfiles=True, allowed_hosts=True, logging=True, secret_key=True):
  9:52:24 AM web.1 |                            ^
  9:52:24 AM web.1 |  SyntaxError: invalid syntax
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3605] [INFO] Worker exiting (pid: 3605)
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3601] [INFO] Shutting down: Master
  9:52:24 AM web.1 |  [2019-02-20 09:52:24 +0000] [3601] [INFO] Reason: Worker failed to boot.
  [DONE] Killing all processes with signal  SIGINT
  9:52:24 AM web.1 Exited with exit code null
  </code></pre>
  </blockquote>
  
  <p>I think that's because heroku is using python command instead of python3, but how can I change that? Please help.   </p>