Merge pull request #1060 from tskit-dev/dependabot/pip/python/requirements/CI-complete/attrs-20.3.0

Bump attrs from 20.2.0 to 20.3.0 in /python/requirements/CI-complete

Author: mergify[bot]

python/requirements/CI-complete/requirements.txt

@@ -1,4 +1,4 @@
-attrs==20.2.0
+attrs==20.3.0
 biopython==1.78
 black==20.8b1
 coverage==5.3