[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/NestedUnionTest.pyi

@@ -0,0 +1,47 @@
+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.NestedUnionTest import NestedUnionTest
+from MyGame.Example.NestedUnion.TestSimpleTableWithEnum import TestSimpleTableWithEnumT
+from MyGame.Example.NestedUnion.Vec3 import Vec3T
+from flatbuffers import table
+
+uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
+
+class NestedUnionTest(object):
+  @classmethod
+  def GetRootAs(cls, buf: bytes, offset: int) -> NestedUnionTest: ...
+  @classmethod
+  def GetRootAsNestedUnionTest(cls, buf: bytes, offset: int) -> NestedUnionTest: ...
+  def Init(self, buf: bytes, pos: int) -> None: ...
+  def Name(self) -> str | None: ...
+  def DataType(self) -> typing.Literal[Any.NONE, Any.Vec3, Any.TestSimpleTableWithEnum]: ...
+  def Data(self) -> table.Table | None: ...
+  def Id(self) -> int: ...
+class NestedUnionTestT(object):
+  name: str | None
+  dataType: typing.Literal[Any.NONE, Any.Vec3, Any.TestSimpleTableWithEnum]
+  data: typing.Union[None, Vec3T, TestSimpleTableWithEnumT]
+  id: int
+  @classmethod
+  def InitFromBuf(cls, buf: bytes, pos: int) -> NestedUnionTestT: ...
+  @classmethod
+  def InitFromPackedBuf(cls, buf: bytes, pos: int = 0) -> NestedUnionTestT: ...
+  @classmethod
+  def InitFromObj(cls, nestedUnionTest: NestedUnionTest) -> NestedUnionTestT: ...
+  def _UnPack(self, nestedUnionTest: NestedUnionTest) -> None: ...
+  def Pack(self, builder: flatbuffers.Builder) -> None: ...
+def NestedUnionTestStart(builder: flatbuffers.Builder) -> None: ...
+def Start(builder: flatbuffers.Builder) -> None: ...
+def NestedUnionTestAddName(builder: flatbuffers.Builder, name: uoffset) -> None: ...
+def NestedUnionTestAddDataType(builder: flatbuffers.Builder, dataType: typing.Literal[Any.NONE, Any.Vec3, Any.TestSimpleTableWithEnum]) -> None: ...
+def NestedUnionTestAddData(builder: flatbuffers.Builder, data: uoffset) -> None: ...
+def NestedUnionTestAddId(builder: flatbuffers.Builder, id: int) -> None: ...
+def NestedUnionTestEnd(builder: flatbuffers.Builder) -> uoffset: ...
+def End(builder: flatbuffers.Builder) -> uoffset: ...
+