Add `.devN` suffixes to all version numbers

[dhermes]

I propose we add .devN suffixes to all version numbers. This way we can accurately reflect, e.g. that google-cloud-pubsub depends on google-cloud-core >= 0.28.0.dev1 rather than a released version of google-cloud-core.

Ref: https://snarky.ca/how-i-manage-package-version-numbers/

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From #4186 (comment)

[lukesneeringer]

I suggested something similar to @jonparrott in chat, although I discussed doing a 1.0 release candidate series.

Let's discuss more tomorrow. I really want to avoid "release the world" pain for customers.

[duggelz]

Could we add the blog link and/or explanantion to CONTRIBUTING.rst? I was looking for this explanation, and it took a bit of googling based on half-remembered commit messages. CC @liyanhui1228

[dhermes]

Sure thing @duggelz. You want to send a PR or you want me, @tseaver, @chemelnucfin or somebody else to send it?

[duggelz]

Since I asked for this, I can write something up, sometime next week.