Bump sphinxcontrib-spelling from 7.2.1 to 7.3.0 (#249)

Bumps [sphinxcontrib-spelling](https://github.com/sphinx-contrib/spelling) from 7.2.1 to 7.3.0.
- [Release notes](https://github.com/sphinx-contrib/spelling/releases)
- [Commits](sphinx-contrib/spelling@7.2.1...7.3.0)

---
updated-dependencies:
- dependency-name: sphinxcontrib-spelling
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

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

requirements/doc-spelling.txt

@@ -1,2 +1,2 @@
 -r doc.txt
-sphinxcontrib-spelling==7.2.1; platform_system!="Windows"  # We only use it in Travis CI
+sphinxcontrib-spelling==7.3.0; platform_system!="Windows"  # We only use it in Travis CI