py3: spawn all python instances with legacy stdio enabled on Windows

This fixes 6 more tests.  See ef7119cd4965.

tests/run-tests.py

@@ -1106,6 +1106,12 b' class Test(unittest.TestCase):'
         # IP addresses.
         env['LOCALIP'] = _strpath(self._localip())
 
+        # This has the same effect as Py_LegacyWindowsStdioFlag in exewrapper.c,
+        # but this is needed for testing python instances like dummyssh,
+        # dummysmtpd.py, and dumbhttp.py.
+        if PYTHON3 and os.name == 'nt':
+            env['PYTHONLEGACYWINDOWSSTDIO'] = '1'
+
         # Reset some environment variables to well-known values so that
         # the tests produce repeatable output.
         env['LANG'] = env['LC_ALL'] = env['LANGUAGE'] = 'C'