Fix the tests to set the serial on the device

The serial is a property of the device, not the key.

Author: arthurdejong

tests/test_misc.doctest

@@ -64,10 +64,10 @@ True
 We can also put device-specific properties in a device:
 
 >>> pskc = PSKC()
->>> device = pskc.add_device(manufacturer='Tokens INC.')
+>>> device = pskc.add_device(manufacturer='Tokens INC.', serial='456')
 >>> len(pskc.keys)
 0
->>> key = device.add_key(id='123', serial='456')
+>>> key = device.add_key(id='123')
 >>> len(pskc.keys)
 1
 >>> key.id

tests/test_write.doctest

@@ -1,6 +1,6 @@
 test_write.doctest - tests for writing PSKC files
 
-Copyright (C) 2014-2024 Arthur de Jong
+Copyright (C) 2014-2025 Arthur de Jong
 
 This library is free software; you can redistribute it and/or
 modify it under the terms of the GNU Lesser General Public
@@ -625,8 +625,8 @@ is not allowed in the RFC 6030 schema. Note that device properties that are
 set on one key end up being applied to both keys.
 
 >>> pskc = PSKC()
->>> device = pskc.add_device(manufacturer='TokenVendorAcme')
->>> key = device.add_key(id='1', serial='123456', secret='1234', counter=42)
+>>> device = pskc.add_device(manufacturer='TokenVendorAcme', serial='123456')
+>>> key = device.add_key(id='1', secret='1234', counter=42)
 >>> key = device.add_key(id='pin0', secret='5678')
 >>> pskc.write(sys.stdout)  #doctest: +ELLIPSIS +REPORT_UDIFF
 <?xml version="1.0" encoding="UTF-8"?>