Add GraalPy support (#5141)

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## 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/implementation.rs

@@ -15,6 +15,7 @@ pub enum ImplementationName {
     #[default]
     CPython,
     PyPy,
+    GraalPy,
 }
 
 #[derive(Debug, Eq, PartialEq, Clone, Ord, PartialOrd, Hash)]
@@ -25,13 +26,14 @@ pub enum LenientImplementationName {
 
 impl ImplementationName {
     pub(crate) fn possible_names() -> impl Iterator<Item = &'static str> {
-        ["cpython", "pypy", "cp", "pp"].into_iter()
+        ["cpython", "pypy", "graalpy", "cp", "pp", "gp"].into_iter()
     }
 
     pub fn pretty(self) -> &'static str {
         match self {
             Self::CPython => "CPython",
             Self::PyPy => "PyPy",
+            Self::GraalPy => "GraalPy",
         }
     }
 }
@@ -50,6 +52,7 @@ impl From<&ImplementationName> for &'static str {
         match v {
             ImplementationName::CPython => "cpython",
             ImplementationName::PyPy => "pypy",
+            ImplementationName::GraalPy => "graalpy",
         }
     }
 }
@@ -73,6 +76,7 @@ impl FromStr for ImplementationName {
         match s.to_ascii_lowercase().as_str() {
             "cpython" | "cp" => Ok(Self::CPython),
             "pypy" | "pp" => Ok(Self::PyPy),
+            "graalpy" | "gp" => Ok(Self::GraalPy),
             _ => Err(Error::UnknownImplementation(s.to_string())),
         }
     }