fix: pin sinon to 7.2.3

Author: greenkeeper[bot]

package.json

@@ -40,7 +40,7 @@
     "react-dom": "16.8.2",
     "react-test-renderer": "16.8.2",
     "rxjs": "^6.4.0",
-    "sinon": "^7.2.3",
+    "sinon": "7.2.3",
     "source-map-loader": "^0.2.4",
     "standard": "^12.0.1",
     "ts-jest": "^23.10.5",