Bump attrs from 18.2.0 to 19.1.0 in /requirements
Bump attrs from 18.2.0 to 19.1.0 in /requirements

Bumps [attrs](https://github.com/python-attrs/attrs) from 18.2.0 to 19.1.0.
- [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/18.2.0...19.1.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
KWierso/treeherder,KWierso/treeherder,edmorley/treeherder,edmorley/treeherder,edmorley/treeherder,KWierso/treeherder,edmorley/treeherder,KWierso/treeherder