Update dependency bignumber.js to v9.1.1

Author: renovate[bot]

package.json

@@ -36,7 +36,7 @@
     "@openzeppelin/contracts-ethereum-package": "2.5.0",
     "@openzeppelin/test-helpers": "0.5.6",
     "@openzeppelin/upgrades": "2.8.0",
-    "bignumber.js": "9.0.1",
+    "bignumber.js": "9.1.1",
     "ethereum-waffle": "2.5.1",
     "ethereumjs-util": "7.0.6",
     "hardlydifficult-eth": "1.1.3",