Add _file_desc_by_toplevel_extension back for compatibility issue with 3.4.1.

_file_desc_by_toplevel_extension is a temporary solution for FieldDescriptor.file.
FieldDescriptor.file was added in code gen. However, we need to keep it for some time for compatibility reason (with 3.4.1 only). May try to remove it after 2 years.

Hi!
Sorry for bothering with such a minor thing, but I just noticed and fixed one annoying typo in WKT's test suite:
`"accpets" -> "accepts"`

Thanks!

Jenkins complains for python 2.6
DEPRECATION: Python 2.6 is no longer supported by the Python core team
ERROR: InvocationError: '/tmp/protobuf/protobuf/python/.tox/py26-cpp/bin/python setup.py -q build_py'
https://grpc-testing.appspot.com/job/protobuf_pull_request/1502/testReport/junit/(root)/python_cpp/python_cpp/

* remove the parens from the cmp() lambda definition

https://github.com/PythonCharmers/python-future/pull/298

* remove the parens from the cmp() lambda definition

https://github.com/PythonCharmers/python-future/pull/298

This reverts commit 703f414c.

* Drop Python 3.3 from testing & add Python 3.5, 3.6

Python 3.3 goes [EOL](https://docs.python.org/devguide/index.html#branchstatus) next month.

* Sync test.sh with the Python versions in tox.ini

* Define cmp() for Python 3

http://python-future.org/compatible_idioms.html?highlight=cmp#cmp

* Define cmp() for Python 3

http://python-future.org/compatible_idioms.html?highlight=cmp#cmp

It seems like the image has a /usr/local/bin/autoconf installed, which
doesn't recognize/work with the yum installed libtools. Putting
distributed binaries/libraries first solves the problem

The old one is deprecated and no longer working.

1, Some tests in reflection_test PY26 raise TypeError but other versions raise ValueError for convert negative long to unsigned
2, Change compare exception type to compare exception str for testDuplicateExtensionNumber. Original code raise 'Double registration of Extensions' is not an instance of (<type 'exceptions.AssertionError'>, <type 'exceptions.ValueError'>) for PY26 cpp implementation

t

* Add scripts to build python wheel for linux.
Followed PEP513.

* Fix dist check for new added files.

* Update README for usage example.

Fix indentation to enable code formatting for sample command lines to set them
visually apart from the surrounding text, and make it easy to copy-paste.

Add code formatting for env vars, paths, binary and library names for
readability.

Hide URLs behind text for readability and conciseness.