Just the API and a bare-bone implementation

Skip SameSeparateTest, SameCommonTest, SplitCommonTest, and
SplitSeparateTest if the interpreter is PyPy.

In _server.py start_server_batch_result is removed because
start_server_batch can only ever fail as a result of a programming
defect in gRPC Python and not the application. This differs from some
analogous-appearing points in _channel.py where the result of
start_client_batch is checked because at those points it is possible
for a failure to indicate a programming defect on the part of the
application.

This should have been removed in, and has done anything but fail on
import since, 4d425522.

client now conforms to spec. See
https://github.com/grpc/grpc/blob/master/doc/negative-http2-interop-test-descriptions.md

This should have been included in 48226a2f.

The fix is more of an elision and the code in the changed area is
reflowed.

This allows inspecting c-level properties of the call after completion.

Using the same namespace results in errors when using the c-extension
version of protobuf due to name collisions

Using the same namespace results in errors when using the c-extension
version of protobuf due to name collisions

This is for compatibility with IPv6-only environments; see the Google
bug b/32531998.

Note that InteropClient.cs and http2_client.cc don't appear to be tested
by our internal branch, but I've included them for consistency.

This brings gRPC Python into conformance with the recommendation in
https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md.