Fix type hints Python 3.9 compatibility

Author: michaelosthege

pymc/sampling/jax.py

@@ -336,7 +336,7 @@ def sample_blackjax_nuts(
     var_names: Optional[Sequence[str]] = None,
     keep_untransformed: bool = False,
     chain_method: str = "parallel",
-    postprocessing_backend: Literal["cpu", "gpu"] | None = None,
+    postprocessing_backend: Optional[Literal["cpu", "gpu"]] = None,
     postprocessing_vectorize: Literal["vmap", "scan"] = "scan",
     idata_kwargs: Optional[Dict[str, Any]] = None,
     postprocessing_chunks=None,  # deprecated
@@ -546,7 +546,7 @@ def sample_numpyro_nuts(
     progressbar: bool = True,
     keep_untransformed: bool = False,
     chain_method: str = "parallel",
-    postprocessing_backend: Literal["cpu", "gpu"] | None = None,
+    postprocessing_backend: Optional[Literal["cpu", "gpu"]] = None,
     postprocessing_vectorize: Literal["vmap", "scan"] = "scan",
     idata_kwargs: Optional[Dict] = None,
     nuts_kwargs: Optional[Dict] = None,