Add "b" to specify bytes at line 82 of stlRW.py

Hey,

Thanks for writing this code, it is very useful! I ran into one bug using the code on line 82 of stlRW.py. The error I got was:
```
error: char format requires a bytes object of length 1
```

This can be fixed with the following change to that line of code (add 'b' to specify bytes):
```
for i in range(0, 80): fout.write(pack("<c", b" "))
```

Thanks again!

Best regards,
Kris