Revert "bump to 2024 edition with rust version 1.85"

This reverts commit 44f3d71.

Author: jonaspleyer

Cargo.toml

@@ -11,8 +11,8 @@ documentation = "https://docs.rs/crate/pyo3/"
 categories = ["api-bindings", "development-tools::ffi"]
 license = "MIT OR Apache-2.0"
 exclude = ["/.gitignore", ".cargo/config", "/codecov.yml", "/Makefile", "/pyproject.toml", "/noxfile.py", "/.github", "/tests/test_compile_error.rs", "/tests/ui"]
-edition = "2024"
-rust-version = "1.85"
+edition = "2021"
+rust-version = "1.63"
 
 [dependencies]
 cfg-if = "1.0"