See https://github.com/grpc/grpc/issues/15253 for more context. The
original behavior when running Python tests is to try to create a
virtual env with the specifed Python version. If there is an issue
with that, fallback to the system's default Python version. This leads to
misleading test results, so removing the fallback and failing the test
when virtual env fails to instantiate the specified Python version is
the new behavior.
murgatroid99