Fix Python3 errors for device tests (#6670)

* Fix Python3 errors for device tests

The Python3 migration didn't include fixes for local scripts in the
device test tree.

Fatal build and run Python errors fixed.

The last update to xunitmerge is ~5 years ago, so it looks to be
unsupported now.

Use a local copy of the two components to allow patching to work with
Python3.

The serial test seems to send garbage chars (non-ASCII/etc.), so use a
codepage 437 which supports all 255 chars.

Fixes: #6660

* Run tests at 160MHz (req'd for some SSL connections)

* Fix debuglevel options for builder

* Fix Python3 interpreter path in xunitmerge

* Remove virtualenv on "make clean"

* Add appropriate attribution, license to xunitmerge

Add like to the original sources with the author's license to the
copied/fixed xunitmerge files.

tests/device/test_ClientContext/test_ClientContext.py

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 from mock_decorators import setup, teardown
 from flask import Flask, request
 from threading import Thread
@@ -21,7 +23,7 @@ def setup_tcpsrv(e):
         sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         for port in range(8266, 8285 + 1):
             try:
-                print >>sys.stderr, 'trying port', port
+                print ('trying port %d' %port, file=sys.stderr)
                 server_address = ("0.0.0.0", port)
                 sock.bind(server_address)
                 sock.listen(1)
@@ -31,17 +33,17 @@ def setup_tcpsrv(e):
                 print >>sys.stderr, 'busy'
         if not running:
             return
-        print >>sys.stderr, 'starting up on %s port %s' % server_address
-        print >>sys.stderr, 'waiting for connections'
+        print ('starting up on %s port %s' % server_address, file=sys.stderr)
+        print ( 'waiting for connections', file=sys.stderr)
         while running:
-            print >>sys.stderr, 'loop'
+            print ('loop', file=sys.stderr)
             readable, writable, errored = select.select([sock], [], [], 1.0)
             if readable:
                 connection, client_address = sock.accept()
                 try:
-                    print >>sys.stderr, 'client connected:', client_address
+                    print('client connected: %s' % str(client_address), file=sys.stderr)
                 finally:
-                    print >>sys.stderr, 'close'
+                    print ('close', file=sys.stderr)
                     connection.shutdown(socket.SHUT_RDWR)
                     connection.close()
 
@@ -54,7 +56,7 @@ def teardown_tcpsrv(e):
     global thread
     global running
     
-    print >>sys.stderr, 'closing'
+    print ('closing', file=sys.stderr)
     running = False
     thread.join()
     return 0