gh-93937: PyOS_StdioReadline() uses PyConfig.legacy_windows_stdio (#94024)

On Windows, PyOS_StdioReadline() now gets
PyConfig.legacy_windows_stdio from _PyOS_ReadlineTState, rather than
using the deprecated global Py_LegacyWindowsStdioFlag variable.

Fix also a compiler warning in Py_SetStandardStreamEncoding().