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

@@ -18,5 +18,6 @@ homepage = "https://binrw.rs"
 license = "MIT"
 publish = false # Use `release.sh`
 repository = "https://github.com/jam1garner/binrw"
+resolver = "3"
 rust-version = "1.66"
 version = "0.15.0-pre"