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/PythonTest.sh

@@ -20,7 +20,7 @@ gen_code_path=${test_dir}
 runtime_library_dir=${test_dir}/../python
 
 # Emit Python code for the example schema in the test dir:
-${test_dir}/../flatc -p -o ${gen_code_path} -I include_test monster_test.fbs
+${test_dir}/../flatc -p -o ${gen_code_path} -I include_test monster_test.fbs --gen-object-api
 
 # Syntax: run_tests <interpreter> <benchmark vtable dedupes>
 #                   <benchmark read count> <benchmark build count>