coq-quickchick.2.0.3 doesn't work on Coq 8.20

[palmskog]

Here's the error message on Coq 8.20+rc1:

File "./src/TacticsUtil.v", line 39, characters 0-1111:
Error:
Non exhaustive match. Values in this pattern are not matched:
Unsafe.String _

cc: @Lysxia