Fix crash with TypeError: 'types.UnionType' object is not callable

[leplatrem]

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/app/telescope/__main__.py", line 6, in <module>
    main(sys.argv[1:])
    ~~~~^^^^^^^^^^^^^^
  File "/app/telescope/app.py", line 640, in main
    checks = Checks.from_conf(conf)
  File "/app/telescope/app.py", line 96, in from_conf
    check = Check(project=project, name=name, **attrs)
  File "/app/telescope/app.py", line 161, in __init__
    self.params[param] = utils.cast_value(_type, value)
                         ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/app/telescope/utils.py", line 491, in cast_value
    return t(value)
TypeError: 'types.UnionType' object is not callable

Makes me see that we don't have integration test of checks