fix: always parse HexTuple files as utf-8

[aucampia]

Summary of changes

Always parse HexTuple files as utf-8 as was the intent anyway as
evidenced by the code that will raise a warning if the encoding provided
for a HexTuple file is something other than utf-8 or None.

rdflib/rdflib/plugins/parsers/hext.py

Lines 73 to 79 in cfa4180

	def parse(self, source, graph, **kwargs):
	if kwargs.get("encoding") not in [None, "utf-8"]:
	warnings.warn(
	f"Hextuples files are always utf-8 encoded, "
	f"I was passed: {kwargs.get('encoding')}, "
	"but I'm still going to use utf-8"
	)

Not adding any tests as this code is already tested and an XFAIL is
removed in this patch.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• 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 remained the same at 90.447% when pulling 8c72b6d on aucampia:iwana-20220803T0022-hext_encoding into cfa4180 on RDFLib:master.

[ghost]

👍