build(deps-dev): Bump postcss-loader from 7.3.0 to 7.3.1

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 7.3.0 to 7.3.1.
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/postcss-loader/compare/v7.3.0...v7.3.1)

---
updated-dependencies:
- dependency-name: postcss-loader
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

package.json

@@ -20,7 +20,7 @@
         "postcss-combine-duplicated-selectors": "^10.0.2",
         "postcss-combine-media-query": "^1.0.1",
         "postcss-import": "^15.1.0",
-        "postcss-loader": "^7.3.0",
+        "postcss-loader": "^7.3.1",
         "postcss-nesting": "^11.2.2",
         "style-loader": "^3.3.3",
         "stylelint": "^15.6.2",