It's unconventional for code to log to the console in .NET.
Even now the logging is reduced, it's still present - especially
in shutdown. This would fix #7361.

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.

gpr_slice_new_with_user_data is like gpr_slice_new, but allows the
caller to specify a distinct pointer to pass to the destroy function.
This is useful when the data is part of a larger data structure that
should be destroyed when the data is no longer needed.