fix: specify rust-version in crate metadata (#3)

Author: jvllmr

Cargo.toml

@@ -7,6 +7,7 @@ readme = "README.md"
 repository = "https://github.com/jvllmr/rustpython-unparser"
 description = "A complete unparser for RustPython ASTs"
 license-file = "LICENSE"
+rust-version = "1.72.1"
 
 [dependencies]
 rustpython-ast = { version = ">=0.4.0" }