pytest needed as dependency/requirement

[ivalaginja]

This is part of openjournals/joss-reviews#7917.

Since you use pytest as your testing suite, this needs to be added to your dependency list and requirements.

Example import here:

PyStarshade/tests/test_bluestein_fft.py

Line 6 in 1af8601

import pytest

[xiaziyna]

Added this and updated the dependencies list in the README!

[ivalaginja]

Checked and it installs it too now. Thanks!