Co-authored-by: Quentin Pradet <quentin.pradet@gmail.com>
Co-authored-by: Ruben Laguna <ruben.laguna@gmail.com>
Co-authored-by: Seth Michael Larson <sethmichaellarson@gmail.com>

Today it's not possible with HTTP/1.1 (see psf/requests#713) and even
python-hyper does not handle this case. This will be handled in
#3336.

As a way to prevent the test suite from making remote calls to anywhere
unexpected, use the `pytest-socket` plugin to disable any calls to
hosts other than localhost.

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

`HTTPResponse._raw_read` raises `IncompleteRead` if the content length does
not match the expected content length. For malformed responses (e.g. 204
response with content) the re-raised `ProtocolError` was a bit too unclear
about the unexpected excess content being the reason for the exception.

With this change, the exception points out, that the client is not dealing
with a connection error but a protocol violation.

While this was done to fix a mypy error, we did not notice the
consequences:

 * This breaks boto3 that subclasses HTTPConnection because
   HTTPHeaderDict does not support bytes values yet.
 * When proxying, headers are still a dictionary by default.

We can decide to reintroduce a forced conversion to HTTPHeaderDict in
urllib3 3.0 if the above issues are fixed.