Modernize Python 2 code to get ready for Python 3

[cclauss]

print() is a function in Python 3.

Make the minimal, safe changes to make the codebase syntax compatible with both Python 2 and Python 3. Being syntax compatible does not mean that the port to Python 3 is complete.

• futurize --stage1 -w . # http://python-future.org/futurize_cheatsheet.html

[jiesutd]

Thanks @cclauss ! you do what I want to do. Although some files have not been updated yet, will merge and update by myself.