Presently it's impossible to use httplib.HTTPConnection.request and send 
the several headers with the same name. This is because _send_request 
assumes a dict is passed in, or a dict-like interface. Obviously one could 
make a list subclass or some such and give it an iteritems that returns 
itself, but IMHO the solution is to fix httplib.

Attached patch changes the current behavior to using iteritems only if it 
exists, otherwise iterate directly (which would fit with sequences of two-
tuples).

This is a feature request for now old versions. It would have to be reformulated as a feature request for a 3.2 module. I do not see the dict (mapping now?) requirement being changed.

Indeed, as Terry wrote the assumption is that header is a mapping (not necessarily a dict). It is not hard to implement a Multimap that has this API:

import collections.abc

class Multimap(collections.abc.Mapping):
    def __init__(self):
        self.data = collections.defaultdict(list)

    def __getitem__(self, key):
        return self.data[key]

    def __setitem__(self, key, value):
        self.data[key].append(value)

    def __iter__(self):
        yield from self.data

    def items(self):
        for k in list(self.data.keys()):
            for v in list(self.data[k]):
                yield (k,v)

    def __len__(self):
        return sum([len(v) for v in self.data.values()])

mm = Multimap()
mm['1'] = 'a'
mm['1'] = 'aa'
mm['1'] = 'aaa'
mm['2'] = 'b'
mm['3'] = 'c'
mm['3'] = 'cc'
print(f'len = {len(mm)}')
print(f'mm.items() = {list(mm.items())}')

Output:
len = 6
mm.items() = [('1', 'a'), ('1', 'aa'), ('1', 'aaa'), ('2', 'b'), ('3', 'c'), ('3', 'cc')]