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/Test.py

@@ -3,6 +3,8 @@
 # namespace: Example
 
 import flatbuffers
+from flatbuffers.compat import import_numpy
+np = import_numpy()
 
 class Test(object):
     __slots__ = ['_tab']
@@ -22,3 +24,34 @@ def CreateTest(builder, a, b):
     builder.PrependInt8(b)
     builder.PrependInt16(a)
     return builder.Offset()
+
+
+class TestT(object):
+
+    # TestT
+    def __init__(self):
+        self.a = 0  # type: int
+        self.b = 0  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        test = Test()
+        test.Init(buf, pos)
+        return cls.InitFromObj(test)
+
+    @classmethod
+    def InitFromObj(cls, test):
+        x = TestT()
+        x._UnPack(test)
+        return x
+
+    # TestT
+    def _UnPack(self, test):
+        if test is None:
+            return
+        self.a = test.A()
+        self.b = test.B()
+
+    # TestT
+    def Pack(self, builder):
+        return CreateTest(builder, self.a, self.b)