Dash 1.0 - Update version to 1.0.0a1

[valentijnnieman]

This PR bumps the version up to 1.0.0-rc1, so that it's in-sync with the other Dash repo's.

Relies on:
dash-renderer 1.0.0
dash-core-components 1.0.0
dash-html-components 1.0.0

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@Marc-Andre-Rivet is hijacking this existing PR from @valentijnnieman 😄

Fixes https://github.com/plotly/dash-core/issues/3

• Bumps version to 1.0.0a1 instead of rc1 to better respect semver
• There's no react deps versions to handle so this is just about tagging the version
• Updated destination branch to release-v1
• Not taking care of dev-requirements.txt yet

[bpostlethwaite]

So what's the story here? I see this thing is targeting release-v1 from 1.0.0-release so does this mean the dash repos will have a release-v1 branch and we'll be printing rc versions from those branches?
Not a blocking comment just curious.

[Marc-Andre-Rivet]

@bpostlethwaite Yes. So while 1.0 is still not ready for prime time we will make development in the release-v1 branch. Once this branch is ready (becomes the new latest/greatest, officially released) we will spin-off a release-v0 branch in case we need to make fixes in v0 and we will merge release-v1 into master.

I'm just picking up the existing PRs and redirecting them / tuning them instead of restarting from scratch when possible.