Allow incompatible requires-python for source distributions with static metadata (#8768)

## Summary

At present, when we have a Python requirement and we see a wheel, we
verify that the Python requirement is compatible with the wheel. For
source distributions, though, we verify that both the Python requirement
_and_ the currently-installed version are compatible, because we assume
that we'll need to build the source distribution in order to get
metadata. However, we can often extract source distribution metadata
_without_ building (e.g., if there's a `pyproject.toml` with no dynamic
keys).

This PR thus modifies the source distribution handling to defer that
incompatibility ("We couldn't get metadata for this project, because it
has no static metadata and requires a higher Python version to run /
build") until we actually try to build the package. As a result, you can
now resolve source distribution-only packages using Python versions
below their `requires-python`, as long as they include static metadata.

Closes https://github.com/astral-sh/uv/issues/8767.

crates/uv-distribution/src/error.rs

@@ -10,7 +10,7 @@ use uv_client::WrappedReqwestError;
 use uv_distribution_filename::WheelFilenameError;
 use uv_fs::Simplified;
 use uv_normalize::PackageName;
-use uv_pep440::Version;
+use uv_pep440::{Version, VersionSpecifiers};
 use uv_pypi_types::{HashDigest, ParsedUrlError};
 
 #[derive(Debug, thiserror::Error)]
@@ -96,6 +96,8 @@ pub enum Error {
     NotFound(Url),
     #[error("Attempted to re-extract the source distribution for `{0}`, but the hashes didn't match. Run `{}` to clear the cache.", "uv cache clean".green())]
     CacheHeal(String),
+    #[error("The source distribution requires Python {0}, but {1} is installed")]
+    RequiresPython(VersionSpecifiers, Version),
 
     /// A generic request middleware error happened while making a request.
     /// Refer to the error message for more details.

crates/uv-distribution/src/source/mod.rs

@@ -1,10 +1,18 @@
 //! Fetch and build source distributions from remote sources.
 
 use std::borrow::Cow;
+use std::ops::Bound;
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::sync::Arc;
 
+use crate::distribution_database::ManagedClient;
+use crate::error::Error;
+use crate::metadata::{ArchiveMetadata, GitWorkspaceMember, Metadata};
+use crate::reporter::Facade;
+use crate::source::built_wheel_metadata::BuiltWheelMetadata;
+use crate::source::revision::Revision;
+use crate::{Reporter, RequiresDist};
 use fs_err::tokio as fs;
 use futures::{FutureExt, TryStreamExt};
 use reqwest::Response;
@@ -26,19 +34,12 @@ use uv_distribution_types::{
 use uv_extract::hash::Hasher;
 use uv_fs::{rename_with_retry, write_atomic, LockedFile};
 use uv_metadata::read_archive_metadata;
+use uv_pep440::release_specifiers_to_ranges;
 use uv_platform_tags::Tags;
 use uv_pypi_types::{HashDigest, Metadata12, RequiresTxt, ResolutionMetadata};
 use uv_types::{BuildContext, SourceBuildTrait};
 use zip::ZipArchive;
 
-use crate::distribution_database::ManagedClient;
-use crate::error::Error;
-use crate::metadata::{ArchiveMetadata, GitWorkspaceMember, Metadata};
-use crate::reporter::Facade;
-use crate::source::built_wheel_metadata::BuiltWheelMetadata;
-use crate::source::revision::Revision;
-use crate::{Reporter, RequiresDist};
-
 mod built_wheel_metadata;
 mod revision;
 
@@ -1798,6 +1799,27 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
     ) -> Result<Option<ResolutionMetadata>, Error> {
         debug!("Preparing metadata for: {source}");
 
+        // Ensure that the _installed_ Python version is compatible with the `requires-python`
+        // specifier.
+        if let Some(requires_python) = source.requires_python() {
+            let installed = self.build_context.interpreter().python_version();
+            let target = release_specifiers_to_ranges(requires_python.clone())
+                .bounding_range()
+                .map(|bounding_range| bounding_range.0.cloned())
+                .unwrap_or(Bound::Unbounded);
+            let is_compatible = match target {
+                Bound::Included(target) => *installed >= target,
+                Bound::Excluded(target) => *installed > target,
+                Bound::Unbounded => true,
+            };
+            if !is_compatible {
+                return Err(Error::RequiresPython(
+                    requires_python.clone(),
+                    installed.clone(),
+                ));
+            }
+        }
+
         // Set up the builder.
         let mut builder = self
             .build_context