test: Use Python 3.6 specific get-pip.py when testing with Python 3.6.
test: Use Python 3.6 specific get-pip.py when testing with Python 3.6.

PiperOrigin-RevId: 425630742

GoogleCloudPlatform/cloud-profiler-python,GoogleCloudPlatform/cloud-profiler-python,GoogleCloudPlatform/cloud-profiler-python,GoogleCloudPlatform/cloud-profiler-python,GoogleCloudPlatform/cloud-profiler-python