ci: migrate to uv! (#4698)

* Migrate to `uv`
* Format `pyproject.toml` files properly
* Remove upper bounds on dependencies, and bounds on dev dependencies
(we should be using latest)
* Move to hatch for packaing

In the future we should:
* Set up dependabot / automate lockfile updates and tests
* Add tests for min compatible versions (I'll do this right after merge)
* Use dynamic versioning
* Bump `pydantic` to v2.11.4 in the lockfile, we have some tests failing

libs/checkpoint-sqlite/tests/test_sqlite.py

@@ -60,7 +60,7 @@ class TestSqliteSaver:
 
     def test_combined_metadata(self) -> None:
         with SqliteSaver.from_conn_string(":memory:") as saver:
-            config = {
+            config: RunnableConfig = {
                 "configurable": {
                     "thread_id": "thread-2",
                     "checkpoint_ns": "",
@@ -70,7 +70,7 @@ class TestSqliteSaver:
             }
             saver.put(config, self.chkpnt_2, self.metadata_2, {})
             checkpoint = saver.get_tuple(config)
-            assert checkpoint.metadata == {
+            assert checkpoint is not None and checkpoint.metadata == {
                 **self.metadata_2,
                 "thread_id": "thread-2",
                 "run_id": "my_run_id",