Cleanup Python 3.8 leftovers (#17967)

Some small cleanups after Python3.8 became EOL.

- Move some type imports from `typing_extensions` to `typing`
- Remove the `abi3-py38` feature from pyo3

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---------

Co-authored-by: Quentin Gliech <quenting@element.io>

rust/Cargo.toml

@@ -34,7 +34,7 @@ pyo3 = { version = "0.23.2", features = [
     "macros",
     "anyhow",
     "abi3",
-    "abi3-py38",
+    "abi3-py39",
 ] }
 pyo3-log = "0.12.0"
 pythonize = "0.23.0"