fix: builtin bytes type missing (#129)

According to pyo3's
[documentation](https://pyo3.rs/v0.23.3/conversions/tables.html#returning-rust-values-to-python),
`Cow<[u8]>` in Rust is Python's `bytes` type, but this mapping is
currently not implemented in the crate.

Co-authored-by: Hiromi Ishii <[email protected]>

Author: Leeeon233

pyo3-stub-gen/src/stub_type/builtins.rs

@@ -59,6 +59,7 @@ impl_builtin!(String, "str");
 impl_builtin!(OsString, "str");
 impl_builtin!(Cow<'_, str>, "str");
 impl_builtin!(Cow<'_, OsStr>, "str");
+impl_builtin!(Cow<'_, [u8]>, "bytes");
 
 impl PyStubType for PathBuf {
     fn type_output() -> TypeInfo {