fix: issues with string destination handling in {Graph,Result}.serialize

[aucampia]

Summary of changes

Change {Graph,Result}.serialize to only handle string destinations as URIs if
the schema is file and to treat it as operating system paths in all
other cases.

This is for the following reasons:

• Result.serialize was treating URI paths as OS paths which only works
  for some cases, for special charachters and percentage encoding it
  does not work.
• Many valid Unix and Windows paths parse using urlparse and have no netloc, e.g. C:\some\path
  and some:/path, however they should be treated as paths and not as URIs.
• Graph and Result should behave consistently.

Some caveats in this change:

• non-file URIs will now be treated as OS paths which may result in
  slightly confusing error messages, such as
  FileNotFoundError: [Errno 2] No such file or directory: 'http://example.com/'
  if http://example.com/ is passed.
• some valid file URIs (e.g. file:/path/to/file from https://datatracker.ietf.org/doc/html/rfc8089)
  are also valid Posix paths but will be treated as file-URIs instead of
  Posix paths. For Graph.serialize this ambiguity can be avoided by
  using pathlib.Path, but for Result.serialize there is currently no
  way to avoid it, though I will work on Make serialize() on a CONSTRUCT result act like normal g.serialize() #1834
  soon and in that provide a way to avoid the ambiguity there also.

• Fixes Result.serialize() path handling is broken for windows paths and some other cases #2067

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.
• For changes that have a potential impact on users of this project:
  • Considered updating our changelog (CHANGELOG.md).
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage decreased (-0.008%) to 90.447% when pulling 4e90415 on aucampia:iwana-20220729T2252-windows_issues into 8d58fd0 on RDFLib:master.

[aucampia]

Not sure what is up with coverage, I'm guessing it is somehow related to the latest build on master failing (https://github.com/RDFLib/rdflib/actions?query=branch%3Amaster), I will rebase once #2068 is merged.

[aucampia]

Not sure what is up with coverage, I'm guessing it is somehow related to the latest build on master failing (https://github.com/RDFLib/rdflib/actions?query=branch%3Amaster), I will rebase once #2068 is merged.

Nevermind, it is because I fiddled with tox.ini when I was trying to debug this.

[ghost]

Huh, windows tax - paid, gj 👍