constants in Python: at the root of the module or in a namespace inside the module?

Well, it depends. Usually, constants are defined at module level. But if you have many constants for category_a and category_b, it might even make sense to add a subpackage constants with modules constants.category_a and constants.category_b.

I would refrain from using a class - it could be instanciated, which wouldn't make sense, and it has no advantage over a module apart from allowing you to cram more than one into one physical file (which you propably shouldn't if there are so many constants). The Java version would propably use a static class, but the Python equivalent is a module.

Name clashes aren't an issue in Python except when you import * - but you shouldn't do that anyway. As long as there are no name clashes inside the module, rest assured that the user will neither pull out all the names from your module into his own nor import it under a name that clashes with another module.

Every module provides its own namespace, so there's no need to create another one.

Having module foo.py:

FOO = 1
BAR = 2
SHMOO = 3

you may use it like this:

import foo
foo.BAR

From style guide: Constants are usually defined on a module level and written in all capital letters with underscores separating words. Examples include MAX_OVERFLOW and TOTAL.