Excel to CSV converter
Good job with the type annotations! Here are a few things you could simplify/improve:


instead of listdir and an extra file extension check, you could use glob.iglob():

for filename in glob.iglob('./**/*.xlsx'):

to improve excel file read performance and memory consumption, you could use read_only=True mode
instead of iterating over sheetnames and getting a sheet by name, you could iterate over available worksheets directly:

for sheet in workbook.worksheets:

iter_rows() should improve the way you read the sheet cells




As a side note and as an idea for an alternative solution: what if you transition the data through a pandas.DataFrame, pandas has .read_excel() and .to_csv() methods.