Bump nox from 2020.8.22 to 2020.12.31 in /.github/workflows (#19)

Bumps [nox](https://github.com/theacodes/nox) from 2020.8.22 to 2020.12.31.
- [Release notes](https://github.com/theacodes/nox/releases)
- [Changelog](https://github.com/theacodes/nox/blob/main/CHANGELOG.md)
- [Commits](wntrblm/nox@2020.8.22...2020.12.31)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/constraints.txt

@@ -1,5 +1,5 @@
 pip==21.0.1
-nox==2020.8.22
+nox==2020.12.31
 nox-poetry==0.5.0
 poetry==1.1.5
 virtualenv==20.1.0