The HTTP/2 specification requires header names to be lower case.

remove skipIfStatement from
TypeSmokeTest.testCallCredentialsFromPluginUpDown since the test passes
on PyPy variants 5.3.0 and newer since these variants have improved
compatibility support for the C-Extensions

Also change grpcio-tools to be a runtime dependency of grpcio-tests,
grpcio-tools is run as part of the tests.

don't run cygrpc_test.TypeSmokeTest.testCallCredentialsFromPluginUpdown
if the interpreter is PyPy

Incremental changes towards PyPy support.

Add a RecordingThreadPool that inherits from Executor, contains a
ThreadPoolExecutor and has an extra method 'was_used' to indicate if
submit method was ever called i.e. if the thread pool was ever used.

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.

-Rename namespace to grpc_health->grpc to match spec
-Proper use of NOT_FOUND status code
-Improve testing
-Add source distribution to artifact build

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.

To make a certain private code base work in a certain way nicer than the
current way.