Bump autoprefixer from 9.7.2 to 9.7.3 (twbs#29787)

Bumps [autoprefixer](https://github.com/postcss/autoprefixer) from 9.7.2 to 9.7.3.
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/master/CHANGELOG.md)
- [Commits](postcss/autoprefixer@9.7.2...9.7.3)

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

package.json

@@ -90,7 +90,7 @@
     "@babel/core": "^7.7.4",
     "@babel/plugin-proposal-object-rest-spread": "^7.7.4",
     "@babel/preset-env": "^7.7.4",
-    "autoprefixer": "^9.7.2",
+    "autoprefixer": "^9.7.3",
     "babel-eslint": "^10.0.3",
     "babel-plugin-istanbul": "^5.2.0",
     "bundlesize": "^0.18.0",