Flatbuffers Python Object API (#5616)

* Flatbuffers Python Object API

Implement the logic to generate the Python object API that can
unpack the data from a buf class into an object class, and pack
the data of an object class to a buf class.

* Fix the build issues

Remove unused parameters and replace auto in the for-loop statement
with std::string to make it compatible with VS2010.

* Fix the build issues.

* Add support for Array type

Added logic to handle Array type in Python Object API. Updated the
generated code accordingly.

* Fix the old style casting from int to char

* Fixed another conversion from int to char

* Fixed the import for typing

Importing typing may cause errors when a machine do not have the
moduel typing installed. This PR fixes the issue by guarding
"import typing" with the "try/except" statement.

* Fix issue of iterating the vector of import list

* Update the generated examples using generate_code.sh

* Fix the import order for typing

The import list was stored in unordered_set, so that each generated
codes may have different import order. Therefore, it failed in the
consistency test where two generated copies need to have exactly the
same apperance.

* Optimize unpack using numpy

Use numpy to unpack vector whenever it is possible to improve unpack
performance.

Also, added codegen command for Python specificly in generate_code.sh,
because --no-includes cannot be turn on for Python.

* Fix the import order

* Update generate_code.bat for windows accordingly

* Replace error message with pass

Avoid printing error message for every Python2 users about typing.
Replace it with pass.

tests/MyGame/Example/Referrable.py

@@ -3,6 +3,8 @@
 # namespace: Example
 
 import flatbuffers
+from flatbuffers.compat import import_numpy
+np = import_numpy()
 
 class Referrable(object):
     __slots__ = ['_tab']
@@ -32,3 +34,35 @@ class Referrable(object):
 def ReferrableStart(builder): builder.StartObject(1)
 def ReferrableAddId(builder, id): builder.PrependUint64Slot(0, id, 0)
 def ReferrableEnd(builder): return builder.EndObject()
+
+
+class ReferrableT(object):
+
+    # ReferrableT
+    def __init__(self):
+        self.id = 0  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        referrable = Referrable()
+        referrable.Init(buf, pos)
+        return cls.InitFromObj(referrable)
+
+    @classmethod
+    def InitFromObj(cls, referrable):
+        x = ReferrableT()
+        x._UnPack(referrable)
+        return x
+
+    # ReferrableT
+    def _UnPack(self, referrable):
+        if referrable is None:
+            return
+        self.id = referrable.Id()
+
+    # ReferrableT
+    def Pack(self, builder):
+        ReferrableStart(builder)
+        ReferrableAddId(builder, self.id)
+        referrable = ReferrableEnd(builder)
+        return referrable