pslab flash is broken since the SerialHandler refactor

[thisisanubhav]

pslab flash <hexfile> fails before it can talk to the bootloader. The refactor in e70d01d ("Refactor SerialHandler") moved ScienceLab to wrap a connection handler (ScienceLab(device), psl.device) and dropped the interface attribute. The flashing code still uses the old API:

1. cli.main calls pslab.ScienceLab(args.port), passing the port string as the device:
   AttributeError: 'str' object has no attribute 'get_firmware_version'. With no -p, it autoconnects a second time, although main has already opened the port with SerialHandler.
2. cli.flash reads psl.interface.baudrate:
   AttributeError: 'ScienceLab' object has no attribute 'interface'
3. ScienceLab.enter_bootloader sets self.device.interface.baudrate:
   AttributeError: 'SerialHandler' object has no attribute 'interface'

flash also passes the ScienceLab itself to mcbootflash, which needs an object with read/write (psl.device has them; ScienceLab doesn't).

Reproduced with a SerialHandler on pyserial's loop:// port:

h = SerialHandler("loop://"); h._ser = serial.serial_for_url("loop://", baudrate=1000000)
psl = ScienceLab.__new__(ScienceLab); psl.device = h
cli.flash(psl, "fw.hex")   # AttributeError: 'ScienceLab' object has no attribute 'interface'
psl.enter_bootloader()      # AttributeError: 'SerialHandler' object has no attribute 'interface'

ScienceLab.read_log has the same leftover (self.device.interface.readline(), self.get_ack()). I haven't included it here, since it's not on the flashing path.