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.
With the upgrade of Kokoro macOS workers to Sierra, not all versions of
Python have a pip version new enough to have TLSv1.2. This change should
make macOS Python testing more resilient to environment changes.
Fix for issue #14815. Pinning to 9.0.3 doesn't work because pip fails to
recognize that as a valid version. Newer versions of pip have a fallback
on macOS to use SecureTransport instead of their outdated OpenSSL that
doesn't support TLSv1.2.
Jan Tattermusch