Merge pull request #284 from python-trio/dependabot/pip/contextlib2-21.6.0

Bump contextlib2 from 0.6.0.post1 to 21.6.0

Author: pquentin

test-requirements.txt

@@ -16,7 +16,7 @@ certifi==2021.5.30
     # via requests
 chardet==4.0.0
     # via requests
-contextlib2==0.6.0.post1
+contextlib2==21.6.0
     # via -r test-requirements.in
 coverage==5.5
     # via pytest-cov