Compile a command and determine whether it is incomplete.
Arguments:
source -- the source string; may contain \n characters
filename -- optional filename from which source was read; default
"<input>"
symbol -- optional grammar start symbol; "single" (default), "exec"
or "eval"
Return value / exceptions raised:
- Return a code object if the command is complete and valid
- Return None if the command is incomplete
- Raise SyntaxError, ValueError or OverflowError if the command is a
syntax error (OverflowError and ValueError can be produced by
malformed literals).