Bump splinter from 0.20.1 to 0.21.0

Bumps [splinter](https://github.com/cobrateam/splinter) from 0.20.1 to 0.21.0.
- [Release notes](https://github.com/cobrateam/splinter/releases)
- [Changelog](https://github.com/cobrateam/splinter/blob/master/docs/news.rst)
- [Commits](cobrateam/splinter@0.20.1...0.21.0)

---
updated-dependencies:
- dependency-name: splinter
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements-dev.txt

@@ -3,7 +3,7 @@ nose2[coverage_plugin]==0.14.0
 cov-core==1.15.0
 black==22.12.0
 pylint==3.0.3
-splinter==0.20.1
+splinter==0.21.0
 selenium==4.16.0
 twine==4.0.2
 Pallets-Sphinx-Themes==2.1.1