winziprint.py: Make steps variable thread safe

src/winziprint.py

@@ -52,12 +52,12 @@ def convert(input_files: list[str],
 
     html_files = [file.lstrip('!') for file in input_files if not file.endswith('.pdf')]
     total_steps = len(html_files) + math.ceil(len(html_files) / BATCH_SIZE) + 1
-    convert.steps = 0
+    steps = [0]
 
     def next_step() -> None:
-        convert.steps += 1
+        steps[0] += 1
         if progress:
-            print(f'progress: {convert.steps}/{total_steps}', file=out, flush=True)
+            print(f'progress: {steps[0]}/{total_steps}', file=out, flush=True)
 
     try:
         tmp_page_nums = []