python: Re-land usage of source file path in toolchain picker (#31893)

This reverts commit 1e55e88c1822402566bdec8a893ec0429d8ee5e6.

Closes #ISSUE

Release Notes:

- Python toolchain selector now uses path to the closest pyproject.toml
as a basis for picking a toolchain. All files under the same
pyproject.toml (in filesystem hierarchy) will share a single virtual
environment. It is possible to have multiple Python virtual environments
selected for disjoint parts of the same project.

crates/repl/src/kernels/mod.rs

@@ -92,7 +92,7 @@ pub fn python_env_kernel_specifications(
     let background_executor = cx.background_executor().clone();
 
     async move {
-        let toolchains = if let Some(toolchains) = toolchains.await {
+        let toolchains = if let Some((toolchains, _)) = toolchains.await {
             toolchains
         } else {
             return Ok(Vec::new());