Test script for pairs::

    >>> from bidict import pairs

Abstracts differences between Python 2 and 3::

    >>> it = pairs({1: 2})
    >>> next(it)
    (1, 2)
    >>> next(it)
    Traceback (most recent call last):
        ...
    StopIteration

Accepts zero or one positional argument which it first tries iterating over
as a mapping (as above), and if that fails, falls back to iterating over as
a sequence, yielding items two at a time::

    >>> it = pairs([(1, 2), (3, 4)])
    >>> next(it)
    (1, 2)
    >>> next(it)
    (3, 4)
    >>> next(it)
    Traceback (most recent call last):
        ...
    StopIteration
    >>> list(pairs())
    []

Mappings may also be passed as keyword arguments, which will be yielded
after any passed via positional argument::

    >>> list(sorted(pairs(a=1, b=2)))
    [('a', 1), ('b', 2)]
    >>> list(sorted(pairs({'a': 1}, b=2, c=3)))
    [('a', 1), ('b', 2), ('c', 3)]
    >>> list(sorted(pairs([('a', 1)], b=2, c=3)))
    [('a', 1), ('b', 2), ('c', 3)]

In other words, this is like a generator analog of the dict constructor.

If any mappings from a sequence or keyword argument repeat an
earlier mapping in the positional argument, repeat mappings will still
be yielded, whereas with dict the last repeat clobbers earlier ones::

    >>> dict([('a', 1), ('a', 2)])
    {'a': 2}
    >>> list(pairs([('a', 1), ('a', 2)]))
    [('a', 1), ('a', 2)]
    >>> dict([('a', 1), ('a', 2)], a=3)
    {'a': 3}
    >>> list(pairs([('a', 1), ('a', 2)], a=3))
    [('a', 1), ('a', 2), ('a', 3)]

Invalid calls result in errors::

    >>> list(pairs(1, 2))
    Traceback (most recent call last):
        ...
    TypeError: Pass at most 1 positional argument (got 2)