Python 3 raises "dictionary size changed during iteration" during parameter modification in Entrez._open

The dict method for `items()` returns an iterator instead of a list in Python 3.  Before trying to delete keys in a loop the items need to be copied to a list.

Here's the error that comes up if any of the params passed to `elink` are `None`:

    File "...lib/python3.3/site-packages/Bio/Entrez/__init__.py", line 224, in elink
        return _open(cgi, variables)
    File "...lib/python3.3/site-packages/Bio/Entrez/__init__.py", line 429, in _open
        for key, value in params.items():
    RuntimeError: dictionary changed size during iteration