fix sonarcloud-reported bug in xmlwriter, add test (#1951)

sonarcloud detected a bug: “Remove 1 unexpected arguments; 'join' expects 1 positional arguments.” in xmlwriter:

```py
return ":".join(pre, uri[len(ns) :])
```

Added containing brackets apparently omitted by original author.

```py
return ":".join([pre, uri[len(ns) :]])
```

Added test for surety, took the opportunity of adding two tests to take coverage of `xmlwriter` to 100%.

rdflib/plugins/serializers/xmlwriter.py

@@ -100,7 +100,7 @@ def qname(self, uri):
         for pre, ns in self.extra_ns.items():
             if uri.startswith(ns):
                 if pre != "":
-                    return ":".join(pre, uri[len(ns) :])
+                    return ":".join([pre, uri[len(ns) :]])
                 else:
                     return uri[len(ns) :]
 

test/test_serializers/test_xmlwriter_qname.py

@@ -0,0 +1,79 @@
+import tempfile
+
+import pytest
+
+import rdflib
+from rdflib.plugins.serializers.xmlwriter import XMLWriter
+
+EXNS = rdflib.Namespace("https://example.org/ns/")
+TRIXNS = rdflib.Namespace("http://www.w3.org/2004/03/trix/trix-1/")
+
+
+def test_xmlwriter_namespaces():
+
+    g = rdflib.Graph()
+
+    with tempfile.TemporaryFile() as fp:
+
+        xmlwr = XMLWriter(fp, g.namespace_manager, extra_ns={"": TRIXNS, "ex": EXNS})
+
+        xmlwr.namespaces()
+
+        fp.seek(0)
+
+        assert fp.readlines() == [
+            b'<?xml version="1.0" encoding="utf-8"?>\n',
+            b'  xmlns:owl="http://www.w3.org/2002/07/owl#"\n',
+            b'  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"\n',
+            b'  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"\n',
+            b'  xmlns:xsd="http://www.w3.org/2001/XMLSchema#"\n',
+            b'  xmlns:xml="http://www.w3.org/XML/1998/namespace"\n',
+            b'  xmlns="http://www.w3.org/2004/03/trix/trix-1/"\n',
+            b'  xmlns:ex="https://example.org/ns/"\n',
+        ]
+
+
+def test_xmlwriter_decl():
+
+    g = rdflib.Graph()
+
+    with tempfile.TemporaryFile() as fp:
+
+        xmlwr = XMLWriter(fp, g.namespace_manager, decl=0, extra_ns={"": TRIXNS})
+
+        xmlwr.namespaces()
+
+        fp.seek(0)
+        assert fp.readlines() == [
+            b"\n",
+            b'  xmlns:owl="http://www.w3.org/2002/07/owl#"\n',
+            b'  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"\n',
+            b'  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"\n',
+            b'  xmlns:xsd="http://www.w3.org/2001/XMLSchema#"\n',
+            b'  xmlns:xml="http://www.w3.org/XML/1998/namespace"\n',
+            b'  xmlns="http://www.w3.org/2004/03/trix/trix-1/"\n',
+        ]
+
+
+@pytest.mark.parametrize(
+    "uri",
+    [
+        # NS bound to “ex”, so “ex:foo”
+        ["https://example.org/ns/foo", "ex:foo"],
+        # NS bound to "", so “graph”
+        ["http://www.w3.org/2004/03/trix/trix-1/graph", "graph"],
+        # NS not in extra_ns, use ns<int> idiom
+        ["https://example.org/foo", "ns1:foo"],
+    ],
+)
+def test_xmlwriter_qname(uri):
+
+    g = rdflib.Graph()
+    g.bind("", TRIXNS)
+    g.bind("ex", EXNS)
+
+    fp = tempfile.TemporaryFile()
+
+    xmlwr = XMLWriter(fp, g.namespace_manager, extra_ns={"": TRIXNS, "ex": EXNS})
+
+    assert xmlwr.qname(uri[0]) == uri[1]