MultiDict class construction regression

[vEpiphyte]

Describe the bug

With multidict 4.7.0, you cannot create a MultiDict with an iterable of key-valu data.

To Reproduce

>>> import sys
>>> import multidict
>>> multidict.__version__
'4.7.0'
>>> sys.version
'3.7.3 (default, Apr 20 2019, 14:54:00) \n[GCC 7.3.0]'
>>> headers=[['User-Agent', 'HTTP Stuff']]
# This is a valid input for the python dict() function
>>> foo = dict(headers)
>>> foo
{'User-Agent': 'HTTP Stuff'}
# Direct MultiDict use failse
>>> multidict.MultiDict(headers)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: MultiDict keys should be either str or subclasses of str
# aiohttp uses CiMultiDict
>>> multidict.CIMultiDict(headers)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: CIMultiDict keys should be either str or subclasses of str

Expected behavior

I expect to make a CIMultiDict / MultiDict with the input list of items.

Logs/tracebacks

Included in the above example.

Your version of the Python

3.7.3 compiled via pyenv on ubuntu 18.04

Your version of the multidict distribution

4.7.0

Additional context

I ran into this with testing aiohttp related calls where headers were being passed as a list of key, value pairs.

aiohttp typedefs their header arguments with a struct they call LooseHeaders defined here https://github.com/aio-libs/aiohttp/blob/v3.6.2/aiohttp/typedefs.py#L42

their docs refer to header arguments as either dicts or list of key/valu pairs https://aiohttp.readthedocs.io/en/stable/client_reference.html#client-session

That is passed into the multidict struct here https://github.com/aio-libs/aiohttp/blob/v3.6.2/aiohttp/client.py#L827

This is valid according to the multidict documentation here:

https://multidict.readthedocs.io/en/stable/multidict.html#multidict

Specifically the line Accepted parameters are the same as for dict.. This is no longer valid with the 4.7.0 release.