This commit cleans up our environment setup for macOS builds.
- Always use `python -m {pip | virtualenv}` form of commands.
- Only upgrade in-place, and never use `--ignore-installed`.
- Use latest tox on macos, and test 3.6 on macos presubmit.
These changes ensure that multibuilder won't break the system-installed Python on the Kokoro macOS builder, and thus won't resolve the ancient system version. For Tox, make sure we test with Python 3.6 (the version of Python in current macOS).
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