Error when workspace contains conflicting Python requirements (#10841)

## Summary

If members define disjoint Python requirements, we should error. Right
now, it seems that it maps to unbounded and leads to weird behavior.

Closes https://github.com/astral-sh/uv/issues/10835.

crates/uv-workspace/src/workspace.rs

@@ -9,6 +9,7 @@ use tracing::{debug, trace, warn};
 use uv_distribution_types::Index;
 use uv_fs::{Simplified, CWD};
 use uv_normalize::{GroupName, PackageName, DEV_DEPENDENCIES};
+use uv_pep440::VersionSpecifiers;
 use uv_pep508::{MarkerTree, VerbatimUrl};
 use uv_pypi_types::{
     Conflicts, Requirement, RequirementSource, SupportedEnvironments, VerbatimParsedUrl,
@@ -383,6 +384,18 @@ impl Workspace {
         conflicting
     }
 
+    /// Returns an iterator over the `requires-python` values for each member of the workspace.
+    pub fn requires_python(&self) -> impl Iterator<Item = (&PackageName, &VersionSpecifiers)> {
+        self.packages().iter().filter_map(|(name, member)| {
+            member
+                .pyproject_toml()
+                .project
+                .as_ref()
+                .and_then(|project| project.requires_python.as_ref())
+                .map(|requires_python| (name, requires_python))
+        })
+    }
+
     /// Returns any requirements that are exclusive to the workspace root, i.e., not included in
     /// any of the workspace members.
     ///