Bump poetry from 1.0.5 to 1.0.8 in /.github/workflows

Bumps [poetry](https://github.com/python-poetry/poetry) from 1.0.5 to 1.0.8.
- [Release notes](https://github.com/python-poetry/poetry/releases)
- [Changelog](https://github.com/python-poetry/poetry/blob/master/CHANGELOG.md)
- [Commits](python-poetry/poetry@1.0.5...1.0.8)

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

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -1,3 +1,3 @@
 pip==20.1.1
 nox==2020.5.24
-poetry==1.0.5
+poetry==1.0.8