sys.stdin not None and missing encoding attribute when running with pythonw.exe

[huguesv]

Environment data

• PTVSD version: 4.2
• OS and version: windows 10
• Python version (& distribution if applicable, e.g. Anaconda): CPython 3.7 using pythonw.exe
• Using VS Code or Visual Studio: VS

Actual behavior

None has no attribute encoding exception

Expected behavior

Either sys.stdin.encoding works, or sys.stdin is None (it is None when running without debugging)

Steps to reproduce:

1. Debug this code using pythonw.exe (no console)

import sys

with open('issue4866.txt', 'wt') as f:
    f.write('hello\n')
    f.write(str(type(sys.stdin)) + '\n')
    if sys.stdin is not None:
        f.write(str(sys.stdin.encoding) + '\n')
    f.write('bye\n')

From microsoft/PTVS#4866