Add missing, undeclared future dependency of celery

[DasSkelett]

Problem

Traceback (most recent call last):
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/kombu/utils/objects.py", line 42, in __get__
    return obj.__dict__[self.__name__]
KeyError: 'backend'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/common.py", line 78, in go
    ret = f(*args, **kw)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/blueprints/api.py", line 126,in wrapper
    return func(*args, **kwargs)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/common.py", line 121, in wrapper
    result = f(*args, **kwargs)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/blueprints/api.py", line 432,in change_password
    send_password_changed(current_user)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/email.py", line 37, in send_password_changed
    send_mail.delay(_cfg('support-mail'), [user.email], f'Your password on {_cfg("site-name")} hasbeen changed',
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/task.py", line 425, in delay
    return self.apply_async(args, kwargs)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/task.py", line 565, in apply_async
    return app.send_task(
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/base.py", line 779, in send_task
    self.backend.on_task_call(P, task_id)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/kombu/utils/objects.py", line 44, in __get__
    value = obj.__dict__[self.__name__] = self.__get(obj)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/base.py", line 1232, in backend
    return self._get_backend()
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/base.py", line 947, in _get_backend
    backend, url = backends.by_url(
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/backends.py", line 74, in by_url
    return by_name(backend, loader), url
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/app/backends.py", line 54, in by_name
    cls = symbol_by_name(backend, aliases)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/kombu/utils/imports.py", line 57, in symbol_by_name
    module = imp(module_name, package=package, **kwargs)
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/lib/python3.8/site-packages/celery/backends/base.py", line 10, in <module>
    from future.utils import raise_with_traceback
ModuleNotFoundError: No module named 'future'

Flask crashes when trying to send an email

Cause

Python's special future module is a new dependency by celery v4.4.4, which they forgot to list in the requirements:
celery/celery#6145
celery/celery#6148
And a fixing PR is already merged, but it may take some time until they release a new version:
celery/celery#6146

Solution

Install the future module manually for now.