chore(deps-dev): update attrs requirement from ~=21.1.0 to ~=21.2.0

Updates the requirements on [attrs](https://github.com/python-attrs/attrs) to permit the latest version.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst)
- [Commits](python-attrs/attrs@21.1.0...21.2.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

dev-requirements.txt

@@ -3,7 +3,7 @@ coverage==5.5
 coveralls==2.2.0
 pytest==6.2.4
 pytest-cov==2.11.1
-attrs~=21.1.0
+attrs~=21.2.0
 Sphinx==3.5.4
 sphinx-bootstrap-theme==0.7.1
 sphinxcontrib-fulltoc==1.2.0