0.19.0 crashes if Flask is not properly installed

[vmarkovtsev]

I've got encode/connexion with aiohttp. When I execute this:

sentry_sdk.init(
        dsn="https://%s@sentry.io/%s" % (sentry_key, sentry_project),
        integrations=[AioHttpIntegration(), SqlalchemyIntegration(),
                      LoggingIntegration(level=logging.INFO, event_level=logging.ERROR),
                      PureEvalIntegration(), ExecutingIntegration()],
    )

I crash with this:

  File "/usr/local/lib/python3.8/dist-packages/sentry_sdk/hub.py", line 106, in _init
    client = Client(*args, **kwargs)  # type: ignore
  File "/usr/local/lib/python3.8/dist-packages/sentry_sdk/client.py", line 86, in __init__
    self._init_impl()
  File "/usr/local/lib/python3.8/dist-packages/sentry_sdk/client.py", line 128, in _init_impl
    self.integrations = setup_integrations(
  File "/usr/local/lib/python3.8/dist-packages/sentry_sdk/integrations/__init__.py", line 119, in setup_integrations
    type(integration).setup_once()
  File "/usr/local/lib/python3.8/dist-packages/sentry_sdk/integrations/flask.py", line 75, in setup_once
    request_started.connect(_request_started)
  File "/usr/local/lib/python3.8/dist-packages/flask/signals.py", line 38, in _fail
    raise RuntimeError(
RuntimeError: Signalling support is unavailable because the blinker library is not installed.

The fact that Flask exists is a random coincidence / an implicit dependency of my project's dependencies, it is not really used.

[vmarkovtsev]

Sorry, I've seen #858