Fix pyqt5_to_pqt6 script when src_to_tokens returns duplicate tokens

Sometimes src_to_tokens returns multiple tokens at the same offset.
This is likely a bug in some versions of tokenize_rt, but this
workaround avoids an exception occurring in the QGIS script.

scripts/pyqt5_to_pyqt6/pyqt5_to_pyqt6.py

@@ -427,6 +427,7 @@ def visit_import(_node: ast.ImportFrom, _parent):
     for i, token in reversed_enumerate(tokens):
         if token.offset in import_offsets:
             end_import_offset = Offset(*import_offsets[token.offset][-2:])
+            del import_offsets[token.offset]
             assert tokens[i].src == 'from'
             token_index = i + 1
             while not tokens[token_index].src.strip():