I initially implemented this for Requests in the requests-toolbelt but
this was already pretty generic because Requests just passes file-like
objects (which is how the streaming encoder behaves) directly to
urllib3. All that needed to change was what we were relying on from the
requests namespace and imports and such.

This also adds the decoder in ther same breath because it's easier to
ensure that's all working together properly in one and it all fits
together nicely.

One thing we _could_ do is consolidate a bunch of the logic too and make
`encode_multipart_formdata` rely on the streaming encoder and call
`getall` instead so that we don't have 2 implementations of the same
logic.

…ta-encoder

Type annotations made it clear that a dangling comma was added by
mistake, converting a value into a tuple.

It's incorrect in the context of the streaming encoder changes.

…a-encoder-2024

# Conflicts:
#	docs/conf.py
#	src/urllib3/connection.py
#	src/urllib3/fields.py
#	src/urllib3/filepost.py
#	test/with_dummyserver/test_connectionpool.py

Previously this test was failing with a warning that was being raised
as an error:

_____________________________________________ TestMultipartEncoder.test_regression_1 ______________________________________________
Traceback (most recent call last):
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/pluggy/_callers.py", line 155, in _multicall
    teardown[0].send(outcome)
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/_pytest/unraisableexception.py", line 88, in pytest_runtest_call
    yield from unraisable_exception_runtest_hook()
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/_pytest/unraisableexception.py", line 78, in unraisable_exception_runtest_hook
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 589, in _callTestMethod
    if method() is not None:
       ^^^^^^^^
ResourceWarning: unclosed file <_io.BufferedReader name='/Users/alexwlchan/repos/urllib3/test/multipart/test_encoder.py'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/_pytest/runner.py", line 341, in from_call
    result: Optional[TResult] = func()
                                ^^^^^^
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/_pytest/runner.py", line 262, in <lambda>
    lambda: ihook(item=item, **kwds), when=when, reraise=reraise
            ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/pluggy/_hooks.py", line 501, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/pluggy/_manager.py", line 119, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/pluggy/_callers.py", line 159, in _multicall
    _warn_teardown_exception(hook_name, teardown[1], e)
  File "/Users/alexwlchan/repos/urllib3/.nox/test-3-12/lib/python3.12/site-packages/pluggy/_callers.py", line 49, in _warn_teardown_exception
    warnings.warn(PluggyTeardownRaisedWarning(msg), stacklevel=5)
pluggy.PluggyTeardownRaisedWarning: A plugin raised an exception during an old-style hookwrapper teardown.
Plugin: unraisableexception, Hook: pytest_runtest_call
PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 589, in _callTestMethod
    if method() is not None:
       ^^^^^^^^
ResourceWarning: unclosed file <_io.BufferedReader name='/Users/alexwlchan/repos/urllib3/test/multipart/test_encoder.py'>

ba64375