Bump attrs from 19.3.0 to 20.3.0 (#235)

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)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements.txt

@@ -11,4 +11,4 @@ tornado==5.1.1; python_version<"3.6"
 tornado==6.0.4; python_version>="3.6"
 trio==0.16.0; python_version>="3.6"
 outcome==1.1.0; python_version>="3.5"
-attrs==19.3.0; python_version>="3.5"
+attrs==20.3.0; python_version>="3.5"