Rename main version branch from 3.0 to 3.x

[acoulton]

As mentioned in #179, it is slightly confusing that the docs version is 3.0 when in fact it will always cover the latest minor of the 3.x version series.

Fixing this would mean:

• Adding a 3.x branch in the repository and configuring that as the default branch
• Configuring RTD to publish a 3.x version
• Configuring RTD to redirect links to the the 3.0 version to the 3.x version
• Disabling the 3.0 version on RTD

@stof do you have access to change the github branch settings?

I can do all the RTD things.