Add GraalPy support (#5141)

<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:

- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->

## Summary

Currently, `uv` refuses to install anything on GraalPy. This is
currently blocking GraalPy testing with cibuildwheel, since manylinux
includes both `uv` and `graalpy` (but doesn't test with `uv`), whereas
cibuildwheel defaults to `uv`. See e.g.
2750618295
where it gives
```
      + python -m build /project/sample_proj --wheel --outdir=/tmp/cibuildwheel/built_wheel --installer=uv
  * Creating isolated environment: venv+uv...
  * Using external uv from /usr/local/bin/uv
  * Installing packages in isolated environment:
    - setuptools >= 40.8.0
  > /usr/local/bin/uv pip install "setuptools >= 40.8.0"
  < error: Unknown implementation: `graalpy`
```

## Test Plan

I simply based the GraalPy support on PyPy and added some small tests.
I'm open to discussing how to test this. GraalPy is available for
manylinux images and with setup-python, so we should be able to add
tests against it to the CI. I locally confirmed by installing `uv` into
a GraalPy venv and then trying things like `uv pip install Pillow` and
testing those extensions.

crates/uv-python/src/discovery.rs

@@ -1710,6 +1710,14 @@ mod tests {
             PythonRequest::parse("pp"),
             PythonRequest::Implementation(ImplementationName::PyPy)
         );
+        assert_eq!(
+            PythonRequest::parse("graalpy"),
+            PythonRequest::Implementation(ImplementationName::GraalPy)
+        );
+        assert_eq!(
+            PythonRequest::parse("gp"),
+            PythonRequest::Implementation(ImplementationName::GraalPy)
+        );
         assert_eq!(
             PythonRequest::parse("cp"),
             PythonRequest::Implementation(ImplementationName::CPython)
@@ -1728,6 +1736,20 @@ mod tests {
                 VersionRequest::from_str("3.10").unwrap()
             )
         );
+        assert_eq!(
+            PythonRequest::parse("graalpy3.10"),
+            PythonRequest::ImplementationVersion(
+                ImplementationName::GraalPy,
+                VersionRequest::from_str("3.10").unwrap()
+            )
+        );
+        assert_eq!(
+            PythonRequest::parse("gp310"),
+            PythonRequest::ImplementationVersion(
+                ImplementationName::GraalPy,
+                VersionRequest::from_str("3.10").unwrap()
+            )
+        );
         assert_eq!(
             PythonRequest::parse("cp38"),
             PythonRequest::ImplementationVersion(
@@ -1749,6 +1771,20 @@ mod tests {
                 VersionRequest::from_str("3.10").unwrap()
             )
         );
+        assert_eq!(
+            PythonRequest::parse("graalpy@3.10"),
+            PythonRequest::ImplementationVersion(
+                ImplementationName::GraalPy,
+                VersionRequest::from_str("3.10").unwrap()
+            )
+        );
+        assert_eq!(
+            PythonRequest::parse("graalpy310"),
+            PythonRequest::ImplementationVersion(
+                ImplementationName::GraalPy,
+                VersionRequest::from_str("3.10").unwrap()
+            )
+        );
 
         let tempdir = TempDir::new().unwrap();
         assert_eq!(
@@ -1819,6 +1855,18 @@ mod tests {
             .to_canonical_string(),
             "pypy@3.10"
         );
+        assert_eq!(
+            PythonRequest::Implementation(ImplementationName::GraalPy).to_canonical_string(),
+            "graalpy"
+        );
+        assert_eq!(
+            PythonRequest::ImplementationVersion(
+                ImplementationName::GraalPy,
+                VersionRequest::from_str("3.10").unwrap()
+            )
+            .to_canonical_string(),
+            "graalpy@3.10"
+        );
 
         let tempdir = TempDir::new().unwrap();
         assert_eq!(