import hypothesis: ModuleNotFoundError: No module named 'attr'

When attempting to import hypothesis, an error occurs on its line:

from hypothesis import given, strategies as st

The error is of a different library attr:

import attr
ModuleNotFoundError: No module named 'attr'

Note: I don't make reference to attr; so I hypothesise that hypothesis is ;).

Terminal installations used include:

pip install hypothesis
pip3 install hypothesis
pip install -r requirements.txt

requirements.txt contains the relevant line:

hypothesis;