Fix manage.py cov Follow-up from "Resolve "Check & Update dependencies 2021-10""

The following discussion from !274 (merged) should be addressed:

@nils.brinckmann started a discussion: (+1 comment)

With the change in the coverage module I run the following into the orchestration repo with this 58-check-update-dependencies-2021-10 branch for the backend:

python3 manage.py cov

And I got an exception:

Traceback (most recent call last):
  File "/usr/src/app/manage.py", line 177, in <module>
    cli()
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1128, in __call__                                                 
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/flask/cli.py", line 600, in main                                                       
    return super().main(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1053, in main                                                     
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1659, in invoke                                                   
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1395, in invoke                                                   
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 754, in invoke                                                    
    return __callback(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/decorators.py", line 26, in new_func                                             
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/flask/cli.py", line 444, in decorator                                                  
    return __ctx.invoke(f, *args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 754, in invoke                                                    
    return __callback(*args, **kwargs)
  File "/usr/src/app/manage.py", line 73, in cov
    COV.stop()
  File "/usr/local/lib/python3.9/site-packages/coverage/control.py", line 575, in stop                                                
    self._collector.stop()
  File "/usr/local/lib/python3.9/site-packages/coverage/collector.py", line 336, in stop                                              
    assert self._collectors[-1] is self, (
AssertionError: Expected current collector to be <Collector at 0x7f12aaf60fa0: CTracer>, but it's <Collector at 0x7f12a6eb9cd0: CTracer>

The tests themselves run just fine (252 tests that passed), but creating the coverage report seems to fail.

I'm not sure if this is due to the updates, so I will retry it with the current master, but I guess this is something we should fix (as long as it is not just me).