[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/MonsterExtra.pyi

@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+import flatbuffers
+import numpy as np
+
+import flatbuffers
+import typing
+from MyGame.MonsterExtra import MonsterExtra
+
+uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
+
+class MonsterExtra(object):
+  @classmethod
+  def GetRootAs(cls, buf: bytes, offset: int) -> MonsterExtra: ...
+  @classmethod
+  def GetRootAsMonsterExtra(cls, buf: bytes, offset: int) -> MonsterExtra: ...
+  @classmethod
+  def MonsterExtraBufferHasIdentifier(cls, buf: bytes, offset: int, size_prefixed: bool) -> bool: ...
+  def Init(self, buf: bytes, pos: int) -> None: ...
+  def D0(self) -> float: ...
+  def D1(self) -> float: ...
+  def D2(self) -> float: ...
+  def D3(self) -> float: ...
+  def F0(self) -> float: ...
+  def F1(self) -> float: ...
+  def F2(self) -> float: ...
+  def F3(self) -> float: ...
+  def Dvec(self, i: int) -> typing.List[float]: ...
+  def DvecAsNumpy(self) -> np.ndarray: ...
+  def DvecLength(self) -> int: ...
+  def DvecIsNone(self) -> bool: ...
+  def Fvec(self, i: int) -> typing.List[float]: ...
+  def FvecAsNumpy(self) -> np.ndarray: ...
+  def FvecLength(self) -> int: ...
+  def FvecIsNone(self) -> bool: ...
+class MonsterExtraT(object):
+  d0: float
+  d1: float
+  d2: float
+  d3: float
+  f0: float
+  f1: float
+  f2: float
+  f3: float
+  dvec: typing.List[float]
+  fvec: typing.List[float]
+  @classmethod
+  def InitFromBuf(cls, buf: bytes, pos: int) -> MonsterExtraT: ...
+  @classmethod
+  def InitFromPackedBuf(cls, buf: bytes, pos: int = 0) -> MonsterExtraT: ...
+  @classmethod
+  def InitFromObj(cls, monsterExtra: MonsterExtra) -> MonsterExtraT: ...
+  def _UnPack(self, monsterExtra: MonsterExtra) -> None: ...
+  def Pack(self, builder: flatbuffers.Builder) -> None: ...
+  def __eq__(self, other: MonsterExtraT) -> bool: ...
+def MonsterExtraStart(builder: flatbuffers.Builder) -> None: ...
+def Start(builder: flatbuffers.Builder) -> None: ...
+def MonsterExtraAddD0(builder: flatbuffers.Builder, d0: float) -> None: ...
+def MonsterExtraAddD1(builder: flatbuffers.Builder, d1: float) -> None: ...
+def MonsterExtraAddD2(builder: flatbuffers.Builder, d2: float) -> None: ...
+def MonsterExtraAddD3(builder: flatbuffers.Builder, d3: float) -> None: ...
+def MonsterExtraAddF0(builder: flatbuffers.Builder, f0: float) -> None: ...
+def MonsterExtraAddF1(builder: flatbuffers.Builder, f1: float) -> None: ...
+def MonsterExtraAddF2(builder: flatbuffers.Builder, f2: float) -> None: ...
+def MonsterExtraAddF3(builder: flatbuffers.Builder, f3: float) -> None: ...
+def MonsterExtraAddDvec(builder: flatbuffers.Builder, dvec: uoffset) -> None: ...
+def MonsterExtraStartDvecVector(builder: flatbuffers.Builder, num_elems: int) -> uoffset: ...
+def StartDvecVector(builder: flatbuffers.Builder, num_elems: int) -> uoffset: ...
+def MonsterExtraAddFvec(builder: flatbuffers.Builder, fvec: uoffset) -> None: ...
+def MonsterExtraStartFvecVector(builder: flatbuffers.Builder, num_elems: int) -> uoffset: ...
+def StartFvecVector(builder: flatbuffers.Builder, num_elems: int) -> uoffset: ...
+def MonsterExtraEnd(builder: flatbuffers.Builder) -> uoffset: ...
+def End(builder: flatbuffers.Builder) -> uoffset: ...
+