store: py26 compat, don't use a dict comprehension

dirs: document performance reasons for bypassing Python C API So someone isn't tempted to change it.

dirs: port PyInt code to work on Python 3 PyIntObject no longer exists in Python 3. Instead, there is PyLongObject. Furthermore, PyInt_AS_LONG is a macro referencing a struct member. PyInt_AS_LONG doesn't exist in Python 3 and PyLong_AS_LONG is a #define for PyLong_AsLong, which is a function. So assigning to the return value of PyLong_AS_LONG doesn't work. This patch introduces a macro for obtaining the value of an integer-like type that works on Python 2 and Python 3. On Python 3, we access the struct field of the underlying PyLongObjet directly, without overflow checking. This is essentially the same as what Python 2 was doing except using a PyLong instead of a PyInt.