Incremental rollout of Python SDK 3.0.0 alpha

[antonpirker]

We are close to releasing the next major update to the Sentry Python SDK 3.0.0. We want to dog food this in sentry.

Because this is a big change we want to roll them out incrementally.

Preparation in sentry-python:

• Release a separate new package sentry-sdk-alpha to PyPI and local PyPI (not part of this PR)

Code changes in sentry:

• Add a feature flag in sentry called something like use-alpha-version-of-python-sdk.
• Fix all call sites of importing the SDK. Something like:

if use_alpha_version_of_python_sdk:
    import sentry_sdk_alpha as sentry_sdk
else:
    import sentry_sdk

• Fix all call sites of start_transaction() because it is gone in 3.0.0:

if use_alpha_version_of_python_sdk:
    start_transaction_func = sentry_sdk.start_transaction
else:
    start_transaction_func = sentry_sdk.start_span

with start_transaction_func(...):
    ...

• Update traces_sampler so it can handle old style parameters and new style parameters. Or make two separate traces_samplers one for each version. See the migration guide to learn what has changed.
• custom_sampling_context does not exist anymore. See the migration guide to learn how to update this.
• Check everything else in the migration guide
• ... probably more ...

Rollout

• first release it just to a very small number of our servers and monitor
• then gradually increase the number of servers this is deployed to.

[antonpirker]

This will not be done. We prepared a PR for dogfooding the new PR and deployed it: getsentry/sentry#92011

The PR was reverted because it uncovered some (not super critical) problems. We will fix those problems and prepare a new dogfooding pr.