Update pyupgrade requirement from ~=2.18 to ~=2.20

Updates the requirements on [pyupgrade](https://github.com/asottile/pyupgrade) to permit the latest version.
- [Release notes](https://github.com/asottile/pyupgrade/releases)
- [Commits](asottile/pyupgrade@v2.18.0...v2.20.0)

---
updated-dependencies:
- dependency-name: pyupgrade
  dependency-type: direct:development
...

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

requirements-dev.txt

@@ -5,6 +5,6 @@ black==21.5b1
 codespell~=2.0
 isort~=5.8
 pydocstyle~=6.1
-pyupgrade~=2.18
+pyupgrade~=2.20
 yamllint~=1.26
 PyGithub~=1.55