Set up `CODEOWNERS` and make review from Python SDK team required

[szokeasaurusrex]

Since the Python SDK team has to maintain all code which gets merged into this repository, an approving review from a member of the team should be required for all PRs before they can be merged into our repository.

Setting this up requires us first to create a CODEOWNERS file, which defines @getsentry/owners-python-sdk as the code owner for everything in the repository.

• meta: Add CODEOWNERS #4182

Then, we can change the branch protection rules to require approval from a code owner on all PRs.