From Anki 2.1.50, the packaged builds include most built-in Python modules. Earlier versions ship with only the standard modules necessary to run Anki.
If your add-on uses a standard Python module that has not been included, or a package from PyPI, then your add-on will need to bundle the module.
For pure Python modules, this is usually as simple as putting them in a subfolder, and adjusting sys.path. For modules that that require C extensions such as numpy, things get a fair bit more complicated, as you'll need to bundle the different module versions for each platform, and ensure you're bundling a version that is compatible with the version of Python Anki is packaged with.