Refactor the Multipart parsing into a Sans-IO layer

[pgjones]

This allows it to be used in async (ASGI) frameworks. It also
hopefully makes the code a little clearer to follow.

This removes the Content-Transfer-Encoding support as RFC7578
states it is deprecated and

Currently, no deployed implementations that send such bodies have
been discovered.

This requires the dataclasses backport, which I think is ok as
dataclasses are in the stdlib, and 3.6 has less than a year till EoL.

• closes Refactor form parsing to be sans-io #1788
• closes werkzeug.formparser is really slow with large binary uploads #875

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

Assuming we work on #1513, this probably isn't necessary any more. It's not intuitive, as it's tracking how much data is currently in the buffer, not how much has been received.

[pgjones]

I'd prefer us to fix #1513 in a separate change, rather than with this (will make this easier for me to manage and hopefully merge). Maybe with the body attribute idea I mentioned.

[davidism]

Nice work, the code is easy for me to follow and well commented. Had a bunch of questions and ideas as I was reviewing.

[davidism]

Seems like this loop would be useful in the sansio class, as a higher level API that yields any available key, value pairs after sending data in. I'm imagining this method could then look something like this:

decoder = MultipartDecoder()

while True:
    data = stream.read(self.buffer_size)
    
    if not data:
        break  # raise if decoder not completed?

    decoder.receive_data(data)

    # iterator stops when complete or waiting for data
    for item in decoder:  # item = (key, value)
        if isinstance(item[1], str):
            fields.append(item)
        else:  # FileStorage
            files.append(item)

    # break if decoder completed?

[pgjones]

This would be a nicer API, however the file data needs to be streamed to disc as it is parsed (say for large files). This prevents it being yielded in a key, value pair as the disc access is IO and hence can not be SansIO.

[pgjones]

Thanks!