Merge pull request #332 from python-trio/dependabot/pip/attrs-21.4.0

Author: pquentin

test-requirements.txt

@@ -8,7 +8,7 @@ alabaster==0.7.12
     # via sphinx
 async-generator==1.10
     # via -r test-requirements.in
-attrs==21.2.0
+attrs==21.4.0
     # via pytest
 babel==2.9.0
     # via sphinx