Refactor Hubs & Scopes

[antonpirker]

See this RFC: getsentry/rfcs#122

There are multiple PRs for this: (For details on the status see the respective PR)

• (0) Cleaning up existing code to prepare for new Scopes API #2611
• (1) Add new scopes related API (but not yet calling it) #2609
• Scope refactoring (merge Hubs and Scopes) #2610

[antonpirker]

Those PRs are merged into the SDK 2.0 branch.

To see what else is missing for the final 2.0 release see all the Issues labeled 2.0: https://github.com/getsentry/sentry-python/issues?q=is%3Aissue+is%3Aopen+label%3A%22SDK+2.0%22