tools: Fix generate-docs-nm-settings-docs-gir.py on Python 2

On python2 the following error is raised:

`LookupError: unknown encoding: unicode`

Seems like `unicode` is a correct encoding in Python 3 but not 2.

Fix:

1. Change encoding to `utf-8`
2. Pass output path string instead of opening file and passing
   opened file object. Python2 and 3 might need different file
   modes, passing just path lets ElementTree select appropriate
   file mode.

Fixes: f00e90923c ('tools: Use ElementTree to write XML in generate-docs-nm-settings-docs-gir.py')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1249

tools/generate-docs-nm-settings-docs-gir.py

@@ -267,12 +267,11 @@ def main(gir_path_str, output_path_str):
                 attrib=property_attributes,
             )
 
-    with open(output_path_str, mode="w") as outfile:
-        docs_gir.write(
-            outfile,
-            encoding="unicode",
-            xml_declaration=True,
-        )
+    docs_gir.write(
+        output_path_str,
+        xml_declaration=True,
+        encoding="utf-8",
+    )
 
 
 if __name__ == "__main__":