Somehow ad hoc descriptions don't work anymore for grpcio (probably
because the README.rst isn't at the distribution root for grpcio).

Before, Cython would *need* to be imported immediately if generated
files did not already exist. Now, missing generated files will trigger a
`setup_requires` inclusion of Cython and defer cythonization until
extension build-time. If cythonization was specified via environment
variable and setup could not find Cython, the extensions are poisoned
instead of blocking non-extension commands from running.

Before we patched the link command, now we just patch `spawn` as an
updatable catch-all solution to ARG_MAX limitations on bash for MSYS and
MinGW and friends.

Removes MinGW flags from various build scripts, since they're now set in
our `setup.py`s by default. Also removes the arguments for the
extensions in grpcio's setup.py; that function was already accessing
globals anyway, might as well go whole-hog.

Before we patched the link command, now we just patch `spawn` as an
updatable catch-all solution to ARG_MAX limitations on bash for MSYS and
MinGW and friends.

The modified link command was originally taken from a Python 2.x distutils.

Even if GRPC_PYTHON_BUILD_WITH_CYTHON is not specified, if the files are
not present then we will fall back to generating with Cython. This
relegates GRPC_PYTHON_BUILD_WITH_CYTHON to providing a regeneration
option rather than being a necessary build environment variable.

Moves all tests into a separate package. This does not change existing
supported means of running tests (e.g. through run_tests.py).

This is needed for building grpcio with mingw, see
https://github.com/grpc/grpc/pull/7012.

Turns out that Python namespace packages may be asymmetrical. The contents of a
particular package that was installed first (here grpcio) appears to retain its
contents in the installation directory when all other distributions declare
that package as a namespace package (here grpcio-tools).

Notable Changes:
  -Convert all str types to byte types at cython layer (ascii encoding)
  -Use six for packages that have different names in Python2/Python3
  -By default, unit tests are compiled/run in Python2.7 and Python3.4
  -Ensure MACOSX_BUILD_TARGET is at least 10.7

Before, namespace packages would break in auditwheel due to a
superfluous check (fixed in the commit referenced in the Dockerfiles).
Now, the auditwheel used in the manylinux1-support Docker images can
handle namespace packages, and we may thus use them. This should
alleviate future user pain w.r.t. installation of grpcio-tools causing
loss of grpcio files and vice versa (e.g. requiring a reinstall of
grpcio following installation of grpcio-tools).