fix: HTTP 308 Permanent Redirect status code handling

[aucampia]

Summary of changes

Change the handling of HTTP status code 308 to behave more like urllib.request.HTTPRedirectHandler, most critically, the new 308 handling will create a new urllib.request.Request object with the new URL, which will prevent state from being carried over from the original request.

One case where this is important is when the domain name changes, for example, when the original URL is http://www.w3.org/ns/adms.ttl and the redirect URL is https://uri.semic.eu/w3c/ns/adms.ttl. With the previous behaviour, the redirect would contain a Host header with the value www.w3.org instead of uri.semic.eu, because the Host header is placed in Request.unredirected_hdrs and takes precedence over the Host header in Request.headers.

Other changes:

• Only handle HTTP status code 308 on Python versions before 3.11 as Python 3.11 will handle 308 by default [ref].

• Move code which uses http://www.w3.org/ns/adms.ttl and http://www.w3.org/ns/adms.rdf out of test_guess_format_for_parse into a separate parameterized test which instead uses the embedded http server.

  This allows the test to fully control the Content-Type header in the response instead of relying on the value that the server is sending.

  This is needed because the server is sending Content-Type: text/plain for the adms.ttl file, which is not a valid RDF format, and the test is expecting Content-Type: text/turtle.

Fixes:

• HTTP 308 handling does not correctly change Host header in request. #2382.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.854% (+0.02%) from 90.83% when pulling 90071df on aucampia:iwana-20230512T1005-fix_test into cbd6151 on RDFLib:main.

[aucampia]

Merging this without reviews, as it is quite well-tested code, and the problem is breaking the CI.

Happy for post-merge feedback if any is forthcoming.