multidict 2.0 release

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.