Replace sys.stderr.write() with print(file=sys.stderr)

[wch]

This addresses #1054.

In some cases, sys.stdout and sys.stderr can be None, which would cause sys.stderr.write() to throw an error. This changes it to use print(file=sys.stderr), which will not throw an error when sys.stderr is None.

From https://docs.python.org/3/library/sys.html#sys.__stderr__:

Under some conditions stdin, stdout and stderr as well as the original values __stdin__, __stdout__ and __stderr__ can be None. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.