uv/tests: update error message

This update changes the error message to one that is worse than the
status quo, but it is still correct because `datasets >= 2.19` doesn't
actually exist given our `EXCLUDE_NEWER` in tests at present.

The underlying cause here seems to be in how PubGrub deals with
reporting incompatibilities. Namely, when it has `foo < 1` and
`foo >= 1`, it reports an incompatibility immediately before looking for
versions. But when it has `foo < 1` and `foo >= 1 ; marker`, then
because they aren't both pubgrub "packages," it starts requesting
versions first and hits the "not available" error path instead of the
"incompatible" error path.

Since this is more of an underlying issue with how we setup
`PubGrubPackage` and our interaction with pubgrub, we ended up deciding
to move forward here with the regression since this PR is fixing a
correctness issue. In particular, if one changes the `requires-python`
to `>=3.8`, then both `main` and this PR produce similarly bad error
messages.

crates/uv/tests/lock.rs

@@ -8662,7 +8662,7 @@ fn unconditional_overlapping_marker_disjoint_version_constraints() -> Result<()>
 
     ----- stderr -----
       × No solution found when resolving dependencies:
-      ╰─▶ Because your project depends on datasets<2.19 and datasets>=2.19, we can conclude that your project's requirements are unsatisfiable.
+      ╰─▶ Because only datasets<2.19 is available and your project depends on datasets>=2.19, we can conclude that your project's requirements are unsatisfiable.
     "###);
 
     Ok(())