[FIX] Credential manager dialog at import

[ales-erjavec]

Issue

At least with PyQt5 5.15.6 on linux simply running

python -c "import orangecontrib.text.widgets.ownyt"

pops-up a Unlock wallet/keychain,... dialog. Rejecting it then prints

Failed to get secret '__pyqtSignature__' of ''Orange3 - NY Times API Key''.
Traceback (most recent call last):
  File "/home/ales/devel/orange3/Orange/widgets/credentials.py", line 42, in __getattr__
    return keyring.get_password(self.service_name, item)
  File "/home/ales/.envs/orange3/lib/python3.10/site-packages/keyring/core.py", line 55, in get_password
    return get_keyring().get_password(service_name, username)
  File "/home/ales/.envs/orange3/lib/python3.10/site-packages/keyring/backends/chainer.py", line 51, in get_password
    password = keyring.get_password(service, username)
  File "/home/ales/.envs/orange3/lib/python3.10/site-packages/keyring/backends/SecretService.py", line 78, in get_password
    collection = self.get_preferred_collection()
  File "/home/ales/.envs/orange3/lib/python3.10/site-packages/keyring/backends/SecretService.py", line 67, in get_preferred_collection
    raise KeyringLocked("Failed to unlock the collection!")
keyring.errors.KeyringLocked: Failed to unlock the collection!

Running orange-canvas for the first time after installing orange3-text or running orange-canvas --force-discovery pops-up four dialogs in succession.

At least with PyQt5 5.15.6 the class scope members are inspected for __pyqtSignature__ attribute which triggers the credential dialog at import/startup time.

Description of changes

Move CredentialManager out of class level scope to instance scope.

Includes

• Code changes
• Tests
• Documentation