v0.6.0

Companion release to aiortp 0.6.0.

• Comfort noise passthrough: CallSession grows cn /
  cn_payload_type, riding through to aiortp.RTPSession.create — with
  cn=True, silence in a call plays RFC 3389 comfort noise instead of
  dead air.
• The rtp extra requires aiortp >= 0.6.0.

No breaking changes from 0.5.0.