python3.* does `print('...')` but not `print '...'`

[jcguu95]

I believe sagemath has completely moved to python3.*, which support print("...") but not print "..." by default. This raises a conflict in the example usage

#+BEGIN_SRC  sage :session foo
  print 'This code will be evaluated in the process buffer *Sage<foo>*.'
#+END_SRC

I think it's better to change it to

#+BEGIN_SRC sage foo.sage
  1+1
#+END_SRC

for the favor of those who are new.

[reverland]

Yeah, example should be updated #7