Learning why module will not load

[Rich Shepard]

Running Slackware-14.2/x86_64 and python-3.9.1. Installed are
six-1.14.0-x86_64-1_SBo and python3-six-1.13.0-x86_64-1_SBo (I don't know if
the latter is required because six is supposed to be available for python2
and python3.)

However, python3 doesn't find either one:

$ python3
Python 3.9.1 (default, Dec 26 2020, 11:21:00) 
[GCC 5.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import six
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'six'
>>> import python3_six
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'python3_six'

How do I diagnose why neither module is found?

TIA,

Rich