Bump PyO3 to 0.20 to fix backtraces (#20517)

Fixes #20460. Caused by PyO3/pyo3#3327.

Nothing in the [changelog](https://pyo3.rs/v0.20.0/changelog.html)
stands out to me as a super-immediate need so doing the minimal upgrade.

src/python/pants/engine/internals/scheduler_test.py

@@ -481,6 +481,8 @@ def test_trace_includes_nested_exception_traceback() -> None:
             return await Get(SomeOutput, SomeInput(outer_input.s))
           File LOCATION-INFO, in __await__
             result = yield self
+          File LOCATION-INFO, in raise_an_exception
+            raise Exception(some_input.s)
         Exception: asdf
 
         The above exception was the direct cause of the following exception:

src/rust/engine/Cargo.toml

@@ -289,7 +289,7 @@ prodash = { git = "https://github.com/stuhood/prodash", rev = "stuhood/raw-messa
 prost = "0.12"
 prost-build = "0.12"
 prost-types = "0.12"
-pyo3 = "0.19"
+pyo3 = "0.20"
 rand = "0.8"
 regex = "1"
 rlimit = "0.8"

src/rust/engine/src/externs/fs.rs

@@ -257,8 +257,8 @@ pub struct PyMergeDigests(pub Vec<DirectoryDigest>);
 #[pymethods]
 impl PyMergeDigests {
     #[new]
-    fn __new__(digests: &PyAny, py: Python) -> PyResult<Self> {
-        let digests: PyResult<Vec<DirectoryDigest>> = PyIterator::from_object(py, digests)?
+    fn __new__(digests: &PyAny, _py: Python) -> PyResult<Self> {
+        let digests: PyResult<Vec<DirectoryDigest>> = PyIterator::from_object(digests)?
             .map(|v| {
                 let py_digest = v?.extract::<PyDigest>()?;
                 Ok(py_digest.0)

src/rust/engine/src/interning.rs

@@ -54,7 +54,7 @@ impl Interns {
         let (id, type_id): (u64, TypeId) = {
             let v = v.as_ref(py);
             let keys = self.keys.as_ref(py);
-            let id: u64 = if let Some(key) = keys.get_item(v) {
+            let id: u64 = if let Some(key) = keys.get_item(v)? {
                 key.extract()?
             } else {
                 let id = self.id_generator.fetch_add(1, atomic::Ordering::Relaxed);