openpyxl can't open xlsx file

Yesterday I've got a very cryptic error.

>>> from openpyxl import load_workbook
>>> wb = load_workbook('super-important.xlsx')

  File ".../python3.9/site-packages/openpyxl/descriptors/base.py", line 128, in __set__
    raise ValueError(self.__doc__)
ValueError: Value must be one of {'lessThanOrEqual', 'lessThan', 'equal', 'greaterThan', 'notEqual', 'greaterThanOrEqual'}

After I placed breakpoint inside openpyxl/descriptors/base.py and run import with debugger I realized that it was complaining about some element of type openpyxl.worksheet.filters.CustomFilter which had value = '**none**'

OOXML specification says that customFilter criteria has Filter Comparison Operator and it should be one of

Valid value	Description
equal	Equal
lessThan	Less Than
lessThanOrEqual	Less Than Or Equal
notEqual	Not Equal
greaterThanOrEqual	Greater Than Or Equal
greaterThan	Greater Than

It seems that my file just does not comply with the OOXML specification.

So the value `**none**` might be supported internally by Excel but it isn't the specification. Supporting the specification is the only sane way to develop the library so I can't blame openpyxl in any way.

<filterColumn colId="2"><customFilters and="true"><customFilter operator="**none**" val=""/></customFilters></filterColumn>

Here is what I found inside my xlsx file in sheet1.xml

I've managed to work around this by perform Format -> Clear direct formatting (Ctrl+M) in LibreOffice.