Bottom border not allowed to be None
*Created originally on Bitbucket by [vitkorob (Victor K)](https://bitbucket.org/%7Bd574a96d-a7c4-44cc-851c-81474ea12378%7D/)* Bottom border not allowed to be None. See line [https://bitbucket.org/openpyxl/openpyxl/src/2.6/openpyxl/styles/borders.py?fileviewer=file-view-default#borders.py-79](https://bitbucket.org/openpyxl/openpyxl/src/2.6/openpyxl/styles/borders.py?fileviewer=file-view-default#borders.py-79) In this code I want only the top border to be thin, and the rest as is. In 2.6.x an exception occurs. ‌ ``` thin_border = Border(top=Side(style='thin'), left=None, right=None, bottom=None, diagonal=None) ``` ‌
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