PR Summary

In NumPy 1.20, the fft is slightly different (more accurate?) and returns 0 as the last entry, instead of some small number ~1e-30. This causes a divide-by-zero warning inlog10. However, this test doesn't really care about the spectrogram, just the frequencies, so we can change the scale to linear to avoid the warning.

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• [n/a] New features are documented, with examples if plot related.
• [n/a] Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• [n/a] New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).