XMLSyntaxError when saving files w/ keep_vba = True

Created originally on Bitbucket by thoboyle (Thomas O'Boyle)

I haven't been able to find if this is simply a bug with version 2.3.x but I am unable to open and successfully save files where keep_vba=True. For example:

wb = openpyxl.load_workbook('doc.xlsm', keep_vba=True) .. wb.save('doc2.xlsm')

fails with:

lxml.etree.XMLSyntaxError: Opening and ending tag mismatch: br line 95 and b, line 96, column 12

even if no changes are made to the file.