[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/Vec3.pyi

@@ -0,0 +1,51 @@
+from __future__ import annotations
+
+import flatbuffers
+import numpy as np
+
+import flatbuffers
+import typing
+from MyGame.Example.NestedUnion.Color import Color
+from MyGame.Example.NestedUnion.Test import Test, TestT
+from MyGame.Example.NestedUnion.Vec3 import Vec3
+
+uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
+
+class Vec3(object):
+  @classmethod
+  def GetRootAs(cls, buf: bytes, offset: int) -> Vec3: ...
+  @classmethod
+  def GetRootAsVec3(cls, buf: bytes, offset: int) -> Vec3: ...
+  def Init(self, buf: bytes, pos: int) -> None: ...
+  def X(self) -> float: ...
+  def Y(self) -> float: ...
+  def Z(self) -> float: ...
+  def Test1(self) -> float: ...
+  def Test2(self) -> typing.Literal[Color.Red, Color.Green, Color.Blue]: ...
+  def Test3(self) -> Test | None: ...
+class Vec3T(object):
+  x: float
+  y: float
+  z: float
+  test1: float
+  test2: typing.Literal[Color.Red, Color.Green, Color.Blue]
+  test3: TestT | None
+  @classmethod
+  def InitFromBuf(cls, buf: bytes, pos: int) -> Vec3T: ...
+  @classmethod
+  def InitFromPackedBuf(cls, buf: bytes, pos: int = 0) -> Vec3T: ...
+  @classmethod
+  def InitFromObj(cls, vec3: Vec3) -> Vec3T: ...
+  def _UnPack(self, vec3: Vec3) -> None: ...
+  def Pack(self, builder: flatbuffers.Builder) -> None: ...
+def Vec3Start(builder: flatbuffers.Builder) -> None: ...
+def Start(builder: flatbuffers.Builder) -> None: ...
+def Vec3AddX(builder: flatbuffers.Builder, x: float) -> None: ...
+def Vec3AddY(builder: flatbuffers.Builder, y: float) -> None: ...
+def Vec3AddZ(builder: flatbuffers.Builder, z: float) -> None: ...
+def Vec3AddTest1(builder: flatbuffers.Builder, test1: float) -> None: ...
+def Vec3AddTest2(builder: flatbuffers.Builder, test2: typing.Literal[Color.Red, Color.Green, Color.Blue]) -> None: ...
+def Vec3AddTest3(builder: flatbuffers.Builder, test3: uoffset) -> None: ...
+def Vec3End(builder: flatbuffers.Builder) -> uoffset: ...
+def End(builder: flatbuffers.Builder) -> uoffset: ...
+