Make pyo3 dependency optional

At the moment, pulling in `outlines-core` with default features disabled
still pulls in, compiles, and pins your version of, `pyo3`.

To fix this, we need to add dependencies which transitively depend on
pyo3 to the optional `python-bindings` dependency set.

Cargo.toml

@@ -12,7 +12,7 @@ once_cell = "1.20"
 thiserror = "2.0"
 pyo3 = { version = "0.22.0", features = ["extension-module"], optional = true }
 regex = "1.10.6"
-serde-pyobject = "0.4.0"
+serde-pyobject = { version = "0.4.0", optional = true }
 serde_json = { version = "1.0", features = ["preserve_order"] }
 serde = {version = "1.0", features = ["derive"]}
 bincode = "2.0.0-rc.3"
@@ -23,7 +23,7 @@ rustc-hash = "2.1.0"
 regex-automata = "0.4.9"
 
 [features]
-python-bindings = ["pyo3"]
+python-bindings = ["pyo3", "serde-pyobject"]
 
 [lib]
 name = "outlines_core"