Replace setup.py with pyproject.toml (#9021)

* Create pyproject.toml
* Implement a custom build backend (see below) in packager directory. Build logic from setup.py has been refactored and migrated into the new backend.
* Tested: pip wheel . (build wheel), python -m build --sdist . (source distribution)

python-package/xgboost/config.py

@@ -16,7 +16,7 @@ def config_doc(
     extra_note: Optional[str] = None,
     parameters: Optional[str] = None,
     returns: Optional[str] = None,
-    see_also: Optional[str] = None
+    see_also: Optional[str] = None,
 ) -> Callable[[_F], _F]:
     """Decorator to format docstring for config functions.
 

python-package/xgboost/plotting.py

@@ -30,7 +30,7 @@ def plot_importance(
     grid: bool = True,
     show_values: bool = True,
     values_format: str = "{v}",
-    **kwargs: Any
+    **kwargs: Any,
 ) -> Axes:
     """Plot importance based on fitted trees.
 
@@ -155,7 +155,7 @@ def to_graphviz(
     no_color: Optional[str] = None,
     condition_node_params: Optional[dict] = None,
     leaf_node_params: Optional[dict] = None,
-    **kwargs: Any
+    **kwargs: Any,
 ) -> GraphvizSource:
     """Convert specified tree to graphviz instance. IPython can automatically plot
     the returned graphviz instance. Otherwise, you should call .render() method
@@ -250,7 +250,7 @@ def plot_tree(
     num_trees: int = 0,
     rankdir: Optional[str] = None,
     ax: Optional[Axes] = None,
-    **kwargs: Any
+    **kwargs: Any,
 ) -> Axes:
     """Plot specified tree.