- version update to 1.3.1
* Fix a race condition in `pick_unused_port()` involving the free ports set.
* Adds an optional `portserver_address` parameter to `pick_unused_port()` so
that callers can specify their own regardless of `os.environ`.
* `pick_unused_port()` now raises `NoFreePortFoundError` when no available port
could be found rather than spinning in a loop trying forever.
* Fall back to `socket.AF_INET` when `socket.AF_UNIX` support is not available
to communicate with a portserver.
* Introduced `add_reserved_port()` and `return_port()` APIs to allow ports to
be recycled and allow users to bring ports of their own.
* Changed default port range to 15000-24999 to avoid ephemeral ports.
* Portserver bugfix.
- convert to single spec
- run test