chore: release v7.0.0a1 + fix release workflow

- Stamp v7.0.0a1 into pyproject.toml + TikTokLive/__version__.py
  (PyPI publish already succeeded for this version; tag below points at this commit)
- Switch the release workflow's "commit version bump" step to open a PR
  instead of pushing directly to master. Direct pushes from
  ``github-actions[bot]`` are rejected by the GH006 protected-branch
  hook. Tag is still pushed directly (tags aren't governed by the
  branch-protection rule).