USB JLink selection by Serial Number

Hello,

I'm porting to PyLink an old Python script that used the JLINKARM.dll directly. Sometimes J-Link selection by serial number is important, the script should report an error and quit if the specific J-Link it should use does not seem to be connected. I thought that could be done at JLink object creation time, but that is not happening. The serial_no argument passed to JLink.__init__() seems to be discarded, if a J-Link with a different serial number is connected to the PC it will be used with no warning whatsoever.

Environment

Windows 10 Pro Version 21H2, Python 2.7.18, JLink 7.20a and PyLink 0.14.3

Steps to reproduce

1. import pylink
2. dbg = pylink.jlink.JLink(serial_no=600115433)
3. dbg.num_connected_emulators()
Output: 1
4. dbg.connected_emulators()
Output: [JLinkConnectInfo(J-Link PLUS <Serial No. 600115434>, Conn. USB)]
5. dbg.open()
6. dbg.serial_number
Output: 600115434

Right now it looks like the only method to actually select a J-Link by serial number is passing the serial_no parameter to the JLink.open() function. For example, if at step 5 something like the following is used:

5*. dbg.open(serial_no=600115433)

an error will be reported and my script will be able to exit with an error code as expected.

Start at JLink.init and compare how its serial_no argument is handled with JLink.open(serial_no=...). Reproduce the mismatch using the connected-emulator and open steps from the issue; done means requesting an unavailable serial number does not silently select a different J-Link and the relevant behavior is covered by tests.