Cells with single quote to escape equal sign

[done712]

Package cannot handle cells in which a single quote is used to escape an equal sign at the beginning of the cell, i.e. in my excel file there is a cell with content "'= this is a test" (please note single quote at the beginning of the string) which raises the error "formulas.errors.FormulaError: ('Not a valid formula:\n%s', '= this is a test')".

[vinci1it2000]

This has been fixed with 844dbf8.

[owinebar]

This is still an issue when exporting a workbook as a dictionary. The single quote escape is not in the exported vale, so when the dictionary is reloaded it's interpreted as a formula.