[Python] Generate .pyi stub files when --python-typing is on. (#8312)

* [Python] Generate `.pyi` stub files when `--python-typing` is on.

To support this change, the following modifications were made:

-  added a new option to disable `numpy` helpers generation;
-  added a new flag to control the target Python version:

   `--python-version` can be one of the following:

   - `0.x.x` – compatible with any Python version;
   - `2.x.x` – compatible with Python 2;
   - `3.x.x` – compatible with Python 3.
-  added codegen utilities for Python;
-  added a note that the generated .py file is empty.

* [Python] Update Bazel build rules.

* [Python] Update Bazel build rules.

* [Python] Run buildifier on BUILD.bazel files.

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/MyGame/Example/NestedUnion/Any.pyi

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+import flatbuffers
+import numpy as np
+
+import flatbuffers
+import typing
+from MyGame.Example.NestedUnion.Any import Any
+from MyGame.Example.NestedUnion.TestSimpleTableWithEnum import TestSimpleTableWithEnum
+from MyGame.Example.NestedUnion.Vec3 import Vec3
+from flatbuffers import table
+
+uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
+
+class Any(object):
+  NONE: int
+  Vec3: int
+  TestSimpleTableWithEnum: int
+def AnyCreator(union_type: typing.Literal[Any.NONE, Any.Vec3, Any.TestSimpleTableWithEnum], table: table.Table) -> typing.Union[None, Vec3, TestSimpleTableWithEnum]: ...
+