Prevent invalid chars in Excel sheet title

rattail/excel.py

@@ -205,6 +205,7 @@ class ExcelWriter(object):
         self.book = openpyxl.Workbook()
         self.sheet = self.book.active
         if sheet_title:
+            sheet_title = sheet_title.replace('/', '-')
             self.sheet.title = sheet_title
         self.number_formats = number_formats
         self.highlight_rows = highlight_rows