Enclosing precedences are wrong in Python code generation

(Possibly?) related: #68

• Here (should be PREC_SUM): https://gitlab.tiker.net/inducer/loopy/blob/06e5163e97dc720f91f1630f34532f0ca659a915/loopy/target/python.py#L268
• And here (should be PREC_IFTHENELSE): https://gitlab.tiker.net/inducer/loopy/blob/06e5163e97dc720f91f1630f34532f0ca659a915/loopy/target/python.py#L144
• These are wrong as well: https://gitlab.tiker.net/inducer/loopy/blob/06e5163e97dc720f91f1630f34532f0ca659a915/loopy/target/python.py#L141-143