Bump splinter from 0.14.0 to 0.15.0

Bumps [splinter](https://github.com/cobrateam/splinter) from 0.14.0 to 0.15.0.
- [Release notes](https://github.com/cobrateam/splinter/releases)
- [Changelog](https://github.com/cobrateam/splinter/blob/master/docs/news.rst)
- [Commits](https://github.com/cobrateam/splinter/compare/0.14.0...0.15.0)

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

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

requirements-dev.txt

@@ -3,7 +3,7 @@ nose2[coverage_plugin]==0.10.0
 cov-core==1.15.0
 pycodestyle==2.7.0
 pylint==2.9.3
-splinter==0.14.0
+splinter==0.15.0
 transifex-client==0.14.2
 selenium==3.141.0
 twine==3.4.1