Build(deps): Bump sphinxcontrib-spelling from 4.3.0 to 5.0.0

Bumps [sphinxcontrib-spelling](https://github.com/sphinx-contrib/spelling) from 4.3.0 to 5.0.0.
- [Release notes](https://github.com/sphinx-contrib/spelling/releases)
- [Commits](sphinx-contrib/spelling@4.3.0...5.0.0)

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

requirements/doc-spelling.txt

@@ -1,2 +1,2 @@
 -r doc.txt
-sphinxcontrib-spelling==4.3.0; platform_system!="Windows"  # We only use it in Travis CI
+sphinxcontrib-spelling==5.0.0; platform_system!="Windows"  # We only use it in Travis CI