Merge pull request #1647 from tskit-dev/dependabot/pip/python/requirements/CI-complete/black-21.8b0

Bump black from 21.7b0 to 21.8b0 in /python/requirements/CI-complete

Author: mergify[bot]

python/requirements/CI-complete/requirements.txt

@@ -1,5 +1,5 @@
 biopython==1.79
-black==21.7b0
+black==21.8b0
 coverage==5.5
 flake8==3.9.2
 h5py==3.4.0