Bump attrs from 20.3.0 to 21.2.0

Bumps [attrs](https://github.com/python-attrs/attrs) from 20.3.0 to 21.2.0.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst)
- [Commits](python-attrs/attrs@20.3.0...21.2.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

dev-requirements.txt

@@ -7,7 +7,7 @@
 
 # -i https://pypi.org/simple
 appdirs==1.4.4
-attrs==20.3.0; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
+attrs==21.2.0; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 black==20.8b1
 click==7.1.2; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'
 coverage==5.3.1