[breaking] Bump Python requirement to 3.10. (#10434)

- Bump the Python requirement.
- Fix type hints.
- Use loky to avoid deadlock.
- Workaround cupy-numpy compatibility issue on Windows caused by the `safe` casting rule.
- Simplify the repartitioning logic to avoid dask errors.

demo/nvflare/horizontal/README.md

@@ -8,7 +8,7 @@ This directory contains a demo of Horizontal Federated Learning using
 To run the demo, first build XGBoost with the federated learning plugin enabled (see the
 [README](../../../plugin/federated/README.md)).
 
-Install NVFlare (note that currently NVFlare only supports Python 3.8):
+Install NVFlare:
 ```shell
 pip install nvflare
 ```

demo/nvflare/vertical/README.md

@@ -8,7 +8,7 @@ This directory contains a demo of Vertical Federated Learning using
 To run the demo, first build XGBoost with the federated learning plugin enabled (see the
 [README](../../../plugin/federated/README.md)).
 
-Install NVFlare (note that currently NVFlare only supports Python 3.8):
+Install NVFlare:
 ```shell
 pip install nvflare
 ```