[BUG]: pybind11::print raises an exception, when started from pythonw

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Problem description

When python is started with pythonw calling pybind11::print raises the AttributeError exception: 'NoneType' object has no attribute 'write'.

The problem is:

• When pythonw is used, sys.stdout has the value None.
• The pybind11 print implementation retrieves the sys.stdout. From that object the write attribute is retrieved and used. Retrieving that write attribute will fail if sys.stdout is None.

The implementation: https://github.com/pybind/pybind11/blob/b07975f492c2eed0409a18353fa23c9969e83e42/include/pybind11/pybind11.h#L2597-L2612

Note: We found the problem on MS windows, and as far as I know pythonw is only available on MS windows.

Reproducible example code

In any c++ code that is called from python, include a call to `pybind11::print`. 

Then call that code via `pythonw`

Start in include/pybind11/pybind11.h around lines 2597-2612 and reproduce the failure with a C++ call to pybind11::print launched through pythonw on Windows. Trace how sys.stdout and its write attribute are retrieved; done means the call no longer raises the reported AttributeError when sys.stdout is None.