Replace list comprehensions with generators (#11)

By using [generator expressions][0] in `_ProxyWriter.writerow` and
`_ProxyWriter.writerows` instead of [list comprehensions][1], those
methods can avoid loading the entire row or iterable of rows into memory
at once, reducing peak memory consumption.

No tests were written due to the difficulty in asserting the behavior
we're looking for is actually happening.

Fixes #10

[0]: https://docs.python.org/3/glossary.html#term-generator-expression
[1]: https://docs.python.org/3/glossary.html#term-list-comprehension

Author: noelleleigh

defusedcsv/csv.py

@@ -48,10 +48,10 @@ def writerow(self, row):
         except TypeError as err:
             msg = "iterable expected, not %s" % type(row).__name__
             raise Error(msg) from err
-        return self.writer.writerow([_escape(field) for field in row])
+        return self.writer.writerow(_escape(field) for field in row)
 
     def writerows(self, rows):
-        return self.writer.writerows([[_escape(field) for field in row] for row in rows])
+        return self.writer.writerows((_escape(field) for field in row) for row in rows)
 
     def __getattr__(self, item):
         return getattr(self.writer, item)