WorkSheet.page_breaks property uninitialized when reading worksheet without breaks
*Created originally on Bitbucket by [Alex Mumme](https://bitbucket.org/%7Bfff3c641-cbc7-478e-a53f-173ca49082ff%7D/)* When reading in an existing workbook containing a sheet without page breaks the ws.page\_breaks property is an empty list. Due to this, any breaks appended to ws.row\_breaks or ws.col\_breaks are ignored when saving the file due to Serialization relying on the tuple that should be in ws.page\_breaks. This reproduces the bug in 3.0.0, but I suspect the bug has existed since commit 2a0fcc9 ‌ ```python import openpyxl as opx from openpyxl.worksheet.pagebreak import Break wb = opx.load_workbook(filename = 'SomeWorkbookWithaSheet1woutPageBreaks.xlsx') ws = wb['Sheet1'] # verify page_breaks is empty list and not 2 val tuple print(f'Collections in page_breaks: {len(ws.page_breaks)}') # add a break, it'll be ignored anyway ws.row_breaks.append(Break(id=5)) # Show there is a break there print(f'Breaks in row_breaks: {ws.row_breaks.count}') # Serializer ignores it wb.save('NewWorkbookAlsowoutPageBreaks.xlsx') # Rename the above to zip, extract, view ./xl/worksheets/sheet1.xml # and note the lack of a <rowBreaks /> element ``` This appears to be due to openpyxl/openpyxl/worksheet/\_reader.py instantiating page\_breaks as \[\], then relying on deserialization to find a rowBreaks or colBreaks element so a break collection can be added.
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