[libcxx] [utils] Improve Python 3 compatibility for merge_archives.py

[broadwaylamb]

Summary

Popen.communicate() method in Python 2 returns a pair of strings, and in Python 3 it returns a pair of byte-like objects unless universal_newlines is set to True. This led to the following error when using Python 3:

Traceback (most recent call last):
  File "<home>/llvm-project/libcxx/utils/merge_archives.py", line 156, in <module>
    main()
  File "<home>/llvm-project/libcxx/utils/merge_archives.py", line 152, in main
    cwd=temp_directory_root, verbose=args.verbose)
  File "<home>/llvm-project/libcxx/utils/merge_archives.py", line 70, in execute_command_verbose
    out, err, exitCode = execute_command(cmd, cwd=cwd)
  File "<home>/llvm-project/libcxx/utils/merge_archives.py", line 58, in execute_command
    p = subprocess.Popen(cmd, **kwargs)
  File "<python>/lib/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "<python>/lib/subprocess.py", line 1119, in _execute_child
    args = list2cmdline(args)
  File "<python>/lib/subprocess.py", line 530, in list2cmdline
    needquote = (" " in arg) or ("\t" in arg) or not arg
TypeError: a bytes-like object is required, not 'str'

With this patch merge_archives.py works fine with Python 3

libcxx/utils/merge_archives.py

 def execute_command(cmd, cwd=None):
     """
     Execute a command, capture and return its output.
     """
     kwargs = {
         'stdin': subprocess.PIPE,
         'stdout': subprocess.PIPE,
         'stderr': subprocess.PIPE,
-        'cwd': cwd
+        'cwd': cwd,
+        'universal_newlines': True
     }
     p = subprocess.Popen(cmd, **kwargs)
     out, err = p.communicate()
     exitCode = p.wait()
     if exitCode == -signal.SIGINT:
         raise KeyboardInterrupt
     return out, err, exitCode