[pre-commit.ci] pre-commit autoupdate

[pre-commit-ci[bot]]

updates:

• https://github.com/ambv/black → https://github.com/psf/black
• https://gitlab.com/pycqa/flake8 → https://github.com/PyCQA/flake8
• github.com/PyCQA/flake8: 3.9.2 → 4.0.1
• https://github.com/pre-commit/mirrors-isort → https://github.com/PyCQA/isort
• github.com/PyCQA/isort: v5.10.1 → 5.10.1

[kmpaul]

The failing tests are due to the hang mentioned in #87. I'm merging because this update should have nothing to do with #87.