Some PyPI releases do not have corresponding tags

[busunkim96]

I did a quick pass through the commit log and it's not completely clear which commit belongs to each tag. Many of the tags were from the days of the 'umbrella' package where all packages were released together.

[tseaver]

I just created tags for the v0.{20,22,23,24,25,26,27,28}.0 releases (v0.21.0 already existed).

I can't find any relevant commit for the PyPI v0.1.0 release: that date would appear to correspond to googleapis/google-cloud-python#2270, which was an umbrella release for google-cloud v0.19.0 (2016-09-19). https://github.com/googleapis/google-cloud-python/tree/4d66524a57d73742b5fe1f4ca571b305031ea5d2/google/cloud/resource_manager had files in it, but the PyPI release is metadata-only (no downloads).

[busunkim96]

Thank you @tseaver!