Gets rid of the deprecation warnings and resolves #437

This replaces dependencies on the stdlib cgi for the multipart package, as recommended by PEP-0594

Unfortunately MultiDict exposes FieldStorage objects publicly, and since that's no longer an option I created a new MultiDictFile object that has the same shape. Beyond that, though, FieldStorage is only an implementation detail and can be swapped out cleanly.

There's one test now failing: test_cgi_escaping_fix. It looks to be special handling of a malformed multipart doc, but I can't figure out the motivation for it. It was originally added in d57f294 and the linked repository no longer exists, so the context is lost.

As an aside: FieldStorage is an extremely weird API. The class represents both a single multipart part and the full multipart document.