www/py-lektor: Change RUN_DEPENDS from py-mistune to py-mistune0

• While I'm here, update version requirement of RUN_DEPENDS
• Bump PORTREVISION for dependency change

This port is not ready for mistune 2.0.0+ which has different API.

Reference: https://github.com/lektor/lektor/blob/v3.2.2/setup.py#L42