update stale version references for 0.9.0

- release_notes.rst: the unreleased section heading is v0.9.0
- installation.rst: pip success message says dawdreamer-0.9.0
- PICKLE_FORMAT.md: the versioning section claimed there was no pickle
  version tracking; describe the pickle_version field and exact-match
  compatibility check that exist in Source/PickleVersion.h