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I propose a fix:
static FILE *
open_exclusive(char *filename, mode_t mode)
{
#if defined(O_EXCL)&&defined(O_CREAT)&&defined(O_WRONLY)&&defined(O_TRUNC)
 /* Use O_EXCL to avoid a race condition when another process tries to
 write the same file. When that happens, our open() call fails,
 which is just fine (since it's only a cache).
 XXX If the file exists and is writable but the directory is not
 writable, the file will never be written. Oh well.
 */
 int fd;
 (void) unlink(filename);
 fd = open(filename, O_EXCL|O_CREAT|O_WRONLY|O_TRUNC
#ifdef O_BINARY
 |O_BINARY /* necessary for Windows */
#endif
#ifdef __VMS
 , mode, "ctxt=bin", "shr=nil"
#elif defined(MS_WINDOWS)
			, mode | _S_IWRITE
#else
 , mode
#endif
 );
 if (fd < 0 )
 return NULL;
 return fdopen(fd, "wb");
#else
 /* Best we can do -- on Windows this can't happen anyway */
 return fopen(filename, "wb");
#endif
}
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so doesn't matter what the .py file permission is under windows, the .pyc file will always have both read and write permissions.