Use a context or tempfile.mkstemp in decorators.py#use_working_copy

[Starou]

SimpleIDML/src/simple_idml/decorators.py

Line 27 in 73fc7d8

tmp_filename = NamedTemporaryFile().name

Followed by https://github.com/Starou/SimpleIDML/blob/master/src/simple_idml/decorators.py#L58

This could be refactored since "Unlike TemporaryFile(), the user of mkstemp() is responsible for deleting the temporary file when done with it."

https://docs.python.org/3.7/library/tempfile.html#tempfile.mkstemp