Portpicker

* No portpicker or portserver code changes
* Fixed the portserver test on recent Python 3.x versions.
* Switched to setup.cfg based packaging.
* We no longer declare ourselves Python 2.7 or 3.3-3.5 compatible.

* 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.

* Renamed portpicker APIs to use PEP8 style function names in code and docs.
* Legacy CapWords API name compatibility is maintained (and explicitly
tested).