Use cargo resolver version 3

once_cell, a transient dependency of trybuild, just released a
minor version update with an increase to their MSRV. Using resolver
3 means that older compiler version with up-to-date cargo (as
exists in CI) will pick compatible dependencies for our MSRV, so it
does not need to be bumped just for the sake of some dev-only
dependency.

Author: csnover

Cargo.toml

@@ -3,7 +3,7 @@ members = [
     "binrw",
     "binrw_derive"
 ]
-resolver = "2"
+resolver = "3"
 
 [workspace.package]
 authors = [