If I am reading this right, then in the below part of the patch, slave_name does not get set before it is used if __MINGW32__ is not defined. this can be fixed by moving the "#endif" up to the line before
    slave_name = ptsname(master_fd); /* get name of slave */



-------

+#if !defined(__MINGW32__)
     sig_saved = PyOS_setsig(SIGCHLD, SIG_DFL);
     /* change permission of slave */
     if (grantpt(master_fd) < 0) {
@@ -4143,10 +4156,15 @@
     slave_name = ptsname(master_fd); /* get name of slave */
     if (slave_name == NULL)
         return posix_error();
+#endif
+#if !defined(__MINGW32__)
     slave_fd = open(slave_name, O_RDWR | O_NOCTTY); /* open slave */
+#else
+    slave_fd = open(slave_name, O_RDWR); /* open slave */
+#endif