[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/Test.pyi

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+import flatbuffers
+import numpy as np
+
+import flatbuffers
+import typing
+from MyGame.Example.NestedUnion.Test import Test
+
+uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
+
+class Test(object):
+  @classmethod
+  def SizeOf(cls) -> int: ...
+
+  def Init(self, buf: bytes, pos: int) -> None: ...
+  def A(self) -> int: ...
+  def B(self) -> int: ...
+class TestT(object):
+  a: int
+  b: int
+  @classmethod
+  def InitFromBuf(cls, buf: bytes, pos: int) -> TestT: ...
+  @classmethod
+  def InitFromPackedBuf(cls, buf: bytes, pos: int = 0) -> TestT: ...
+  @classmethod
+  def InitFromObj(cls, test: Test) -> TestT: ...
+  def _UnPack(self, test: Test) -> None: ...
+  def Pack(self, builder: flatbuffers.Builder) -> None: ...
+
+def CreateTest(builder: flatbuffers.Builder, a: int, b: int) -> uoffset: ...
+