It's fairly safe to assume the terminal is 79 characters wide (that's where the python line length suggestion comes from), so you can pad using 79 as the benchmark. If that's not good enough, then you could use shutil to get the actual width of the terminal.
>>> width = 79
>>> text_left = "help()"
>>> text_right = "Display this help message."
>>> remaining_space = width - len(text_left) - len(text_right)
>>> remaining_space
47
>>> print("{0}{1}{2}".format(text_left, ("."*remaining_space), text_right))
help()...............................................Display this help message.
>>> import shutil
>>> width = shutil.get_terminal_size().columns
>>> remaining_space = width - len(text_left) - len(text_right)
>>> width
120
>>> remaining_space
88
>>> print("{0}{1}{2}".format(text_left, ("."*remaining_space), text_right))
help()........................................................................................Display this help message.
>>>