fixed some Python statement regarded as shell command

I try the example play file:
>>> python kryten.py -d 0.05 -n -p python101.play

It turns out that I get the following errors:

....5> a = 'hello world'
/bin/sh: a: command not found
....6> a = [1,2,3]
/bin/sh: a: command not found
....7> b = a
/bin/sh: b: command not found
....8> print a, b
     | Traceback (most recent call last):
     |   File "kryten.py", line 675, in play
     |     if not actor(command,code,history,options):
     |   File "kryten.py", line 545, in actor
     |     exec(command,_env)
     |   File "<string>", line 1, in <module>
     | NameError: name 'a' is not defined
....9> b[1] = 7
     | Traceback (most recent call last):
     |   File "kryten.py", line 675, in play
     |     if not actor(command,code,history,options):
     |   File "kryten.py", line 545, in actor
     |     exec(command,_env)
     |   File "<string>", line 1, in <module>
     | NameError: name 'b' is not defined
...10> print a, b
     | Traceback (most recent call last):
     |   File "kryten.py", line 675, in play
...

I tryed it on my Mac OS X, so I thought it might be a OS specific thing. Then I tryed the same script on my Ubuntu, I get the same error. 

It turns out that "a=4" works fine, "a = 4" fails because it is regarded as a shell command.

And when "print 5", I see nothing on the terminal, finally I see "set_color('black')", and my terminal's background color is black, oops, maybe add some options?