Add support for upgrading Python in tool environments (#7605)

This PR adds support for upgrading the build environment of tools with
the addition of a ```--python``` argument to ```uv upgrade```, as
specified in #7471.

Some things to note:
- I added support for individual packages — I didn't think there was a
good reason for ```--python``` to only apply to all packages
- Upgrading with ```--python``` also upgrades the package itself — I
think this is fair as if a user wants to _strictly_ switch the version
of Python being used to build a tool's environment they can use ```uv
install```. This behavior can of course be modified if others don't
agree!

Closes https://github.com/astral-sh/uv/issues/6297.

Closes https://github.com/astral-sh/uv/issues/7471.

crates/uv-cli/src/lib.rs

@@ -3396,6 +3396,20 @@ pub struct ToolUpgradeArgs {
     #[arg(long, conflicts_with("name"))]
     pub all: bool,
 
+    /// Upgrade a tool, and specify it to use the given Python interpreter
+    /// to build its environment. Use with `--all` to apply to all tools.
+    ///
+    /// See `uv help python` for details on Python discovery and supported
+    /// request formats.
+    #[arg(
+        long,
+        short,
+        env = "UV_PYTHON",
+        verbatim_doc_comment,
+        help_heading = "Python options"
+    )]
+    pub python: Option<String>,
+
     #[command(flatten)]
     pub installer: ResolverInstallerArgs,