Improve display of Python versions (#1029)

In https://github.com/astral-sh/puffin/pull/986 there was some confusion
about what these values are set to and I noticed that we never actually
display the target version being used for a resolution.

- Consistently display the Python interpreter being used, i.e. make it
clear that we are referring the the interpreter/installed Python version
and always show the version number
- Display the target Python version during solving

crates/puffin-resolver/src/resolver/mod.rs

@@ -257,6 +257,11 @@ impl<'a, Provider: ResolverProvider> Resolver<'a, Provider> {
             FxHashMap::default();
         let mut next = root;
 
+        debug!(
+            "Solving with target Python version {}",
+            self.python_requirement.target()
+        );
+
         loop {
             // Run unit propagation.
             state.unit_propagation(next)?;