Error raise when use eval() to calculate expression string contained 0 arity function terminal

[liningbo]

deap 1.3.1
python 3.8

def if_then_else(input, output1, output2):
    return output1 if input else output2

def no_input_func_1():
    return 2.0

def no_input_func_2():
    return True

pset = gp.PrimitiveSetTyped('main', [bool, float], float)

pset.addPrimitive(operator.xor, [bool, bool], bool)
pset.addPrimitive(operator.mul, [bool, bool], bool)
pset.addPrimitive(if_then_else, [bool, float, float], float)
pset.addTerminal(3.0, float)
pset.addTerminal(True, bool)

pset.renameArguments(ARG0 = 'x')
pset.renameArguments(ARG1 = 'y')

pset.addTerminal(no_input_func_1,float)
pset.addTerminal(no_input_func_2,bool)

the expression string of a PrimitiveTree will be like if_then_else(x, 3.0, no_input_func_1) which will raise error when use eval() to calculate it, because of no_input_func_1 can not be executed but no_input_func_1() can.

so I think 0 arity function terminal should be formated as no_input_func_1() not no_input_func_1.

I have created a merge request which I have used well.
#643