drop invalid XML element attributes

Fixes issue #375

The bug happened when we had a `:` in an element attribute that didn't match any XML namespace (invalid XML). In the example it was `padding:1px=""; margin:15px=""`
We can workaround it by manually dropping those bad elements.
I hope it doesn't impact performance too much

To reproduce:
`trafilatura -u  https://web.archive.org/web/20230619162141/https://www.tristatetelecom.com/productdetailI2.aspx?dataid=IPGSM-4G --xml`

Minimal reproduction example:
```
echo 'Testing<ul style="" padding:1px; margin:15px""><b>Features:</b> <li>Saves the cost of two dedicated phone lines.</li> al station using Internet or cellular technology.</li> <li>Requires no change to the existing Fire Alarm Control Panel configuration. The IPGSM-4G connects directly to the primary and secondary telephone ports.</li>
' | trafilatura --xml
```

tests/unit_tests.py

@@ -387,7 +387,10 @@ def test_external():
         teststring = f.read()
     assert extract(teststring, no_fallback=True, include_tables=False) == ''
     assert extract(teststring, no_fallback=False, include_tables=False) == ''
-
+    # invalid XML attributes: namespace colon in attribute key (issue #375). Those attributes should be stripped
+    bad_xml = 'Testing<ul style="" padding:1px; margin:15px""><b>Features:</b> <li>Saves the cost of two dedicated phone lines.</li> al station using Internet or cellular technology.</li> <li>Requires no change to the existing Fire Alarm Control Panel configuration. The IPGSM-4G connects directly to the primary and secondary telephone ports.</li>'
+    res = extract(bad_xml, output_format='xml')
+    assert "Features" in res
 
 def test_images():
     '''Test image extraction function'''

trafilatura/utils.py

@@ -303,6 +303,12 @@ def sanitize_tree(tree):
         preserve_space = elem.tag in SPACING_PROTECTED or parent_tag in SPACING_PROTECTED
         trailing_space = elem.tag in FORMATTING_PROTECTED or parent_tag in FORMATTING_PROTECTED or preserve_space
 
+        for attrib_key in elem.attrib.keys():
+            # Remove invalid attributes
+            if ':' in attrib_key:  # colon is reserved for namespaces in XML
+                if not elem.attrib[attrib_key] or attrib_key.split(':')[0] not in tree.nsmap:
+                    elem.attrib.pop(attrib_key)
+
         if elem.text:
             elem.text = sanitize(elem.text, preserve_space, trailing_space)
         if elem.tail: