serial.serialutil.SerialException: Attempting to use a port that is not open

Trying to use sniffer.py on macOS Sierra (10.12.1) is giving errors:
```
$ sudo python sniffer.py /dev/tty.usbserial-DJ00IO3W
Password:
Logging data to logs/capture.pcap
Connecting to sniffer on /dev/tty.usbserial-DJ00IO3W
Scanning for BLE devices (5s) ...
Traceback (most recent call last):
File "sniffer.py", line 156, in
devlist = scanForDevices()
File "sniffer.py", line 50, in scanForDevices
mySniffer.scan()
File "/Users/itemir/Temp/Adafruit_BLESniffer_Python/SnifferAPI/Sniffer.py", line 62, in scan
self._startScanning()
File "/Users/itemir/Temp/Adafruit_BLESniffer_Python/SnifferAPI/SnifferCollector.py", line 206, in _startScanning
self._packetReader.sendScan()
File "/Users/itemir/Temp/Adafruit_BLESniffer_Python/SnifferAPI/Packet.py", line 186, in sendScan
self.sendPacket(REQ_SCAN_CONT, [], timeout)
File "/Users/itemir/Temp/Adafruit_BLESniffer_Python/SnifferAPI/Packet.py", line 183, in sendPacket
self.uart.writeList(packetString, timeout)
File "/Users/itemir/Temp/Adafruit_BLESniffer_Python/SnifferAPI/UART.py", line 73, in writeList
nBytes = self.ser.write(array)
File "/Library/Python/2.7/site-packages/serial/serialposix.py", line 497, in write
raise portNotOpenError
serial.serialutil.SerialException: Attempting to use a port that is not open
```
[ble-sniffer-osx](http://sourceforge.net/projects/nrfblesnifferosx/) from Roland King works, so it is not an OS issue nor a hardware issue.

It also looks like there have been others who ran into this issue. See the following thread: https://forums.adafruit.com/viewtopic.php?f=22&t=82839&sid=643613138ca6829f5444dd61c1cabc7d&start=15 (Workaround mentioned there, to install 2.3 FTDI driver, does also not work).

Reproduce the command on macOS, then trace the failing path from sniffer.py through SnifferAPI/Sniffer.py, SnifferCollector.py, Packet.py, and UART.py. Check the serial-port lifecycle around the write in UART.py and compare the behavior with the referenced ble-sniffer-osx workaround; done means scanning no longer raises the port-not-open exception.

Written by the indexing model from the issue text.