Bump attrs from 20.2.0 to 20.3.0

Bumps [attrs](https://github.com/python-attrs/attrs) from 20.2.0 to 20.3.0.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/python-attrs/attrs/compare/20.2.0...20.3.0)

requirements.txt

-attrs==20.2.0
+attrs==20.3.0
 Click==7.1.2
 cycler==0.10.0
 dataclasses==0.6

sphinx-doc/requirements_sphinx.txt

 alabaster==0.7.12
 appdirs==1.4.4
 astor==0.8.1
-attrs==20.2.0
+attrs==20.3.0
 Babel==2.8.0
 black==20.8b1
 certifi==2020.6.20