Run Windows against Python 3.13 (#2171)

## Summary

In Python 3.13, at least in the current builds, there's no `python.exe`,
but there is `venvlauncher.exe`.

I've asked here about whether it's intended:
https://discuss.python.org/t/when-should-venv-scripts-nt-python-exe-be-present/47620.
But there's at least some evidence in CPython
[here](d457345bbc/Lib/venv/__init__.py (L270))
that we should fall back to these, and the tests pass.

Closes https://github.com/astral-sh/uv/issues/1636.

.github/workflows/system-install.yml

@@ -61,8 +61,8 @@ jobs:
       - name: "Validate global Python install"
         run: python3.11 scripts/check_system_python.py --uv ./target/debug/uv
 
-  install-windows:
-    name: "Install Python on Windows"
+  install-windows-python-310:
+    name: "Install Python 3.10 on Windows"
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v4
@@ -82,9 +82,41 @@ jobs:
       - name: "Print Python path"
         run: echo $(which python)
 
+      - name: "Create virtual environment"
+        run: ./target/debug/uv venv
+
       - name: "Validate global Python install"
         run: py -3.10 ./scripts/check_system_python.py --uv ./target/debug/uv
 
+  install-windows-python-313:
+    name: "Install Python 3.13 on Windows"
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+          allow-prereleases: true
+          cache: pip
+
+      - name: "Install Rust toolchain"
+        run: rustup show
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: cargo build
+
+      - name: "Print Python path"
+        run: echo $(which python)
+
+      - name: "Create virtual environment"
+        run: ./target/debug/uv venv
+
+      - name: "Validate global Python install"
+        run: py -3.13 ./scripts/check_system_python.py --uv ./target/debug/uv
+
   install-pyenv:
     name: "Install Python using pyenv"
     runs-on: ubuntu-latest

crates/uv-virtualenv/src/bare.rs

@@ -176,7 +176,26 @@ pub fn create_bare_venv(
                 .join("scripts")
                 .join("nt")
                 .join(python_exe);
-            fs_err::copy(shim, scripts.join(python_exe))?;
+            match fs_err::copy(shim, scripts.join(python_exe)) {
+                Ok(_) => {}
+                Err(err) if err.kind() == io::ErrorKind::NotFound => {
+                    // If `python.exe` doesn't exist, try the `venvlaucher.exe` shim.
+                    let shim = interpreter
+                        .stdlib()
+                        .join("venv")
+                        .join("scripts")
+                        .join("nt")
+                        .join(match python_exe {
+                            "python.exe" => "venvwlauncher.exe",
+                            "pythonw.exe" => "venvwlauncher.exe",
+                            _ => unreachable!(),
+                        });
+                    fs_err::copy(shim, scripts.join(python_exe))?;
+                }
+                Err(err) => {
+                    return Err(err.into());
+                }
+            }
         }
     }
     #[cfg(not(any(unix, windows)))]