Former version crashed in case of syntax error
(cf test_syntax_error.py)

The goal is to reduce the memory consumption.

Make possible to read the output from the process lauching the process.

This commit detect when a statement is evaluated with PyRun_String run
with Py_eval_input. This seems to generate a Syntax Error. Thus if
an exception with Syntax Error is generated with Py_eval_input,
the python interpreter does a second pass with Py_single_input.
The string err is set appropriatly, out has to be checked.

    This is required in order to make import of module dl succeed.

       This function is equivalent to print() under python,
       is equivalent to str() in dg-python.

	  Executable dg-python emulates a minimal python interpreter.
	  The interpreter is able to
	    - execute single statements: (import, print,...),
	    - evaluate expressions.
	  Expressions and statements are separated by semi-colons. Several
	  statements and/or evaluations between two semi-colons yield an
	  undefined behavior.