Compatible with Python 3

[florentx]

The map built-in in Python 3 returns a <map object>, and it crashes with a ValueError.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./terminalplot/terminalplot.py", line 21, in plot
    (max(y) - min(y)) if y and max(y) - min(y) != 0 else rows
ValueError: min() arg is an empty sequence

[kressi]

thank you