[jorenham]

This fixes the following three stubtest errors:

av.buffer.Buffer is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.buffer.Buffer.__buffer__ is inconsistent, stub parameter "flags" should be positional-only (add "/", e.g. "flags, /")
av.buffer.ByteSource is not present in stub

Stubtest also complains about a missing av.buffer.__pyx_capi__, but I wasn't sure if it'd be worth the churn, so I left it out 🤷