Note that this is a PR on the py3k branch.

I did a runthrough of the Python 3 lessons after merging @rgaiacs's PR to clean up any loose ends. After making these changes, the lesson runs properly for me in Python 3.

Often, these changes are just noticing when Python 3 gives different outputs than Python 2 and updating accordingly, and a few spots where we still had print statement (these were added after #142 was made).

One odd thing is that in Python 3, the list returned by glob isn't sorted. I'm not sure if this is also true in Python 2, but to be sure that the output is correct, I added sorted calls.

Another somewhat large change is that the exception classes are changed in Python 3; importantly, there's no longer a base IOError. Instead, the specific IOErrors are separated out. Updated that lesson accordingly.

Things I noticed but didn't change:

1. I found many floats that truncated differently in the lessons (e.g., 126.5) that truncated differently in my Python 3 environment (e.g., 126.50000000000001). This might be something specific to my machine though, not necessarily Python 3?
2. In 02-loop, the challenge "From 1 to N" mentions that range(3) produces [0, 1, 2]. In Python 3 it instead produces an object, which prints as range(0, 3). You can still iterate over it though, so it doesn't directly affect the challenge, but the pedant in me has a "well, actually" moment when I read the challenge.
3. In Lesson 6, the difference in standard deviations before and after is exactly zero on my machine. I'm not sure if this is, again, a machine difference, or if it's a Python 3 difference.