Allow Node tests to run with or without UV, change default version to 7, add some portability tests. Also make some more core tests work with libuv

A top-level "grpc" package is added and RPC Framework
is made public.

The source code is moved from src/python to
src/python/src. A setup.py is added at
src/python. The build_python.sh and
run_python.sh scripts are updated to build
and run the Python tests by building a
package and installing it in the developer's
Python 2.7 virtual environment.