[ENH] Print warning when type is unkown in pure-python memoryview

[matusvalo]

Is your feature request related to a problem? Please describe.

Consider following cython code:

# cython is *not* imported
def foo(bar: cython.int[:]):
    pass

This code is compiled successfully without any warning even though cython.int type is ignored.

Describe the solution you'd like.

When annotation type is not recognized a warning should be raised:

warning: deleteme.py:2:19: Unknown type declaration 'cython.int' in annotation, ignoring

This is the same behavior as we have already for simple types:

# cython is *not* imported
def foo(bar: cython.int):
    pass

Describe alternatives you've considered.

Alternative is to keep as it is but this can end up with stupid mistakes as mine - see issue #5649

Additional context

No response