Make merge_opt_params compatible with Python 3

[austinbutler]

In Python 3 iteritems does not exist, so I was getting an AttributeError.

The command used was s4cmd dsync --recursive --delete-removed --API-Metadata='{"Cache-Control": "max-age=31536000"}' build <my_bucket>

[Exception] 'dict' object has no attribute 'iteritems'
Exception in thread Thread-10:d(s)]
Traceback (most recent call last):
  File "/Users/abutler/.local/pipx/venvs/s4cmd/bin/s4cmd.py", line 520, in run
    self.__class__.__dict__[func_name](self, *args, **kargs)
  File "/Users/abutler/.local/pipx/venvs/s4cmd/bin/s4cmd.py", line 129, in wrapper
    ret = func(*args, **kargs)
  File "/Users/abutler/.local/pipx/venvs/s4cmd/bin/s4cmd.py", line 1336, in upload
    'privilege': self.get_file_privilege(source)})
  File "/Users/abutler/.local/pipx/venvs/s4cmd/bin/s4cmd.py", line 399, in wrapped_method
    merged_kargs = self.merge_opt_params(method, kargs)
  File "/Users/abutler/.local/pipx/venvs/s4cmd/bin/s4cmd.py", line 432, in merge_opt_params
    for k, v in getattr(self.opt, key).iteritems():
AttributeError: 'dict' object has no attribute 'iteritems'