Pin dependency prop-types to 15.7.2 (#145)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

package-lock.json

@@ -38,7 +38,7 @@
     "polka": "0.5.2",
     "preact": "10.5.14",
     "prettier": "2.3.2",
-    "prop-types": "^15.7.2",
+    "prop-types": "15.7.2",
     "react": "17.0.2",
     "react-dom": "17.0.2",
     "remark-cli": "9.0.0",