Program that provides diagrams for documentation
The Doxygen recommendation notwithstanding,

Consider adding PEP484 type hints, particularly to the signatures of functions such as draw_diagram. For that method, it's not really correct Python terminology to call the return value &quot;void&quot; - it's None.
Does the single-expression statement diagram.models actually have any effect? If so that deserves a comment.
Rather than glob.glob consider pathlib.Path.glob which has some usage niceties
path.basename(f)[:-3] is a very fragile way of acquiring a stem. Again, pathlib.Path can do a better job.

The line
dict([(name, cls) for name, cls in module.__dict__.items() if isinstance(cls, type)])

first of all should not use an inner list, since dict can directly accept a generator; and secondly would be better-represented as a dictionary comprehension:
{name: cls for name, cls in module.__dict__.items() if isinstance(cls, type)}

This loop:
    for name, cls in classes.items():
        print(f'Drawing {module_name}.{name} diagram')
        draw_diagram(module, cls_name=name, output_path=path.join(doc_path, module_name))

doesn't actually use cls, so should only iterate over the dictionary's keys, not its items.