Multidict

* Fix 99: raise `RuntimeError` on dict iterations if the dict was changed

* Update `__init__.pyi` signatures

Changes
-------------

* Refactor internal data structures: main dict operations are about
100% faster now.

* Preserve order on multidict updates 68

Updates are `md[key] = val` and `md.update(...)` calls.

Now **the last** entry is replaced with new key/value pair, all
previous occurrences are removed.

If key is not present in dictionary the pair is added to the end

* Force keys to `str` instances 88

* Implement `.popall(key[, default])` 84

* `.pop()` removes only first occurence, `.popone()` added 92

* Implement dict's version 86

* Proxies are not pickable anymore 77

Changes
------------

* binary wheels for python 3.6 are added

CHANGES
- Fix `CIMultiDict` constructor for case of accepting `istr` 11

CHANGES
- Allow to create proxy from proxy
- Add type hints (PEP-484)

The major multidict release.

New version is slightly incompatible with multidict==1.x

We have changed _canonical representation_ for case insensitive strings.

Changes
- `upstr` is renamed to `istr` (case insensitive string)
- `upstr` is remained as alias for `istr`
- `istr` uses `str.title()` for converting into canonical form.
It means `istr('content-length')` now represented as `Content-Length` instead of `CONTENT-LENGTH`.
- All tests that depend on `repr(upstr(...))` are broken, everything else should work as expected.

P.S.
Sorry for inconvenience but the change was requested many times.
At least _canonical_ representation of HTTP headers (the main consumer of the library) is title-cased.

_aiohttp_ library will utilize new representation in next major release.