utils: export_excel.py: Reimplement the `save_virtual_workbook` function

In the latest openpyxl, `openpyxl.writer.excel.save_virtual_Workbook` method is deprecated.

utils/export_excel.py

 import itertools
+from tempfile import NamedTemporaryFile
 
 from openpyxl import Workbook
 from openpyxl.styles import Alignment, Border, Font, Side
-from openpyxl.writer.excel import save_virtual_workbook
+from openpyxl.writer.excel import save_workbook
+
+def save_virtual_workbook(workbook):
+    with NamedTemporaryFile() as f:
+        save_workbook(workbook, f)
+        f.seek(0)
+        return f.read()
 
 _BD = Side(style='thin', color="000000")
 _CENTER = Alignment(