build(deps): bump attrs from 19.3.0 to 20.3.0

Bumps [attrs](https://github.com/python-attrs/attrs) from 19.3.0 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](python-attrs/attrs@19.3.0...20.3.0)

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

Author: dependabot[bot]

requirements/base.txt

@@ -31,7 +31,7 @@ signxml==2.8.0
 #       - lxml
 #       - pyOpenSSL
 #           - six
-attrs==19.3.0
+attrs==20.3.0
 certifi==2020.4.5.1
 cffi==1.14.0
 eight==1.0.0