Implements #3380

* Add kokoro config for building python wheels on windows

* Move script to root

* Debug

* Debug

* Change working directory

* Add msbuild to path

* Debug

* Fix python dir name

* Debug

* Move artifacts to targeted directory

* Update artifact directory

* Debug

* Debug

* Automatic generation of version

* Reduce file numbers

* Add config for building python wheel on mac

* Specify os name

* Clean up venv

* Debug

* Recover old path

* Add config for building python wheel

* Update submodules

* Fix setup.py to use c++11

* Fix syntax error

* Fix syntax error

* Add dir for artifact

* Update artifact dirctory

* Remove python 3.3

* Clean up virtual env

* Clean up env

* Test environment

* Test env variable

* Automatic update version number

* Remove dependency on python-wheel branch

Backport building C# nuget to 3.6.x branch

This reverts commit c02346c72aafd1826b0397dbbce4ce04bf252f50.

* Check the message to be encoded is the wrong type. (#4885)

* Change TypeError to ArgumentError

This change only makes assertEquals pass for message in c extension. However, it actually does nothing.
This is the same behavior before 3.6.0 release.

Cherry-pick master build fixes and kokoro changes to 3.6.x

* Add continuous test for ruby 2.3, 2.4 and 2.5

* Change ruby 2.5 to 2.5.0

* No need to provide argument to rb_funcall when argc is 0

* Fix tests for ruby 2.5

* Use rescue instead of assert_raise to accept subclass of error

This avoids the need to use "yum update && yum upgrade" in the container
to be able to contact GitHub, which requires TLS 1.2[1].

I have verified that binaries built with this container still run in the
previous container; no errors like "/lib64/libc.so.6: version
`GLIBC_2.14' not found", which occur if using too new of a glibc when
compiling. CentOS 6.6 has glibc version 2.12 release 1.209.el6. CentOS
6.9 has glibc version 2.12 release 1.149.el6. Both would upgrade to
release 1.212.el6 via "yum update && yum upgrade".

1. https://githubengineering.com/crypto-deprecation-notice/

Now that we depend on C++11, we need at least GCC 4.8 instead of 4.7.
This change updates the Docker setup to continue using CentOS 6.6 but
with GCC 4.8.

I also added libm to the whitelist for dynamically linked libraries for
the ARM64 protoc binary.

* Remove js_embed binary.

1. Added C++ tests under the release docker image.
2. Cleaned up tests.sh and removed traivis stuff.

1. Changed maven script to only do artifact uploading and removed build
script invocation from it. We didn't use maven to invoke the build
script before (we built protoc manually and editted pom.xml to only do
uploading for previous releases), and will not use it in the future (we
will use kokoro to build artifacts).
2. Cleaned up build-protoc.sh and README.md: removed the part about
using maven to build and listed supported platforms explicitly.

Add kokoro bazel configs to 3.6.x branch.

Additional support for building and deploying ppcle_64 artifacts