fixed system keyring authentification issue

- Removed superfluous call to `_call_keyring`
- Replaced deprecated `imp` module

src/caosdb/connection/authentication/keyring.py

@@ -28,7 +28,7 @@ retrieve the password.
 """
 
 import sys
-import imp
+import importlib
 from getpass import getpass
 from caosdb.exceptions import ConfigurationError
 from .external_credentials_provider import ExternalCredentialsProvider
@@ -52,17 +52,13 @@ def get_authentication_provider():
 
 def _get_external_keyring():
     try:
-        fil, pathname, desc = imp.find_module("keyring", sys.path[1:])
-        module = imp.load_module("external_keyring", fil, pathname, desc)
-        return module
+        return importlib.import_module("keyring")
     except ImportError:
         raise RuntimeError(
             "The keyring password method requires installation of the"
             "keyring python package. On linux with python < 3.5, "
             "this requires the installation of dbus-python as a "
             "system package.")
-    finally:
-        fil.close()
 
 
 def _call_keyring(**config):
@@ -74,7 +70,6 @@ def _call_keyring(**config):
     url = config.get("url")
     username = config.get("username")
     app = "caosdb — {}".format(url)
-    password = _call_keyring(app=app, username=username)
     external_keyring = _get_external_keyring()
     password = external_keyring.get_password(app, username)
     if password is None: