Close file handles opened by signature functions that starts with ‘i’

for csv, csvz file formats, file handles will be left open. for xls, ods file formats, the file is read all into memory and is close afterwards. for xlsx, file handles will be left open in python 2.7 - 3.5 by pyexcel-xlsx(openpyxl). In other words, pyexcel-xls, pyexcel-ods, pyexcel-ods3 won’t leak file handles.