A new common funcion:assert_never to assert certain part of the code is never reached.

Following instructions of typing and mypy:
https://typing.readthedocs.io/en/latest/guides/unreachable.html#assert-never-and-exhaustiveness-checking
https://mypy.readthedocs.io/en/stable/literal_types.html#id3

Co-authored-by: Alexander Goscinski <alex.goscinski@posteo.de>

Author: khsrali

src/aiida/common/asserts.py

@@ -0,0 +1,26 @@
+###########################################################################
+# Copyright (c), The AiiDA team. All rights reserved.                     #
+# This file is part of the AiiDA code.                                    #
+#                                                                         #
+# The code is hosted on GitHub at https://github.com/aiidateam/aiida-core #
+# For further information on the license, see the LICENSE.txt file        #
+# For further information please visit http://www.aiida.net               #
+###########################################################################
+"""Module with asserts that can be used in the codebase."""
+
+from typing import Optional
+
+try:
+    from typing import Never
+except ImportError:
+    # Fallback for Python 3.10 and older
+    from typing_extensions import Never
+
+
+def assert_never(arg: Never, message: Optional[str] = None) -> Never:
+    """Assert a part of code that should never be reached.
+    This is useful, especially when adding new variables to an enum,
+    so to remind the developers to handle the new variable in all the switch cases.
+    """
+
+    raise AssertionError('This part of code should never be reached' if not message else message)