Matching Python 2 int behavior on Python 2 (#1186)

Pybind11's default conversion to int always produces a long on Python 2 (`int`s and `long`s were unified in Python 3). This patch fixes `int` handling to match Python 2 on Python 2; for short types (`size_t` or smaller), the number will be returned as an `int` if possible, otherwise `long`. Requires Python 2.5+.

This is needed for things like `sys.exit`, which refuse to accept a `long`.

tests/test_builtin_casters.py

@@ -323,3 +323,16 @@ def test_numpy_bool():
     assert convert(np.bool_(False)) is False
     assert noconvert(np.bool_(True)) is True
     assert noconvert(np.bool_(False)) is False
+
+
+def test_int_long():
+    """In Python 2, a C++ int should return a Python int rather than long
+    if possible: longs are not always accepted where ints are used (such
+    as the argument to sys.exit()). A C++ long long is always a Python
+    long."""
+
+    import sys
+    must_be_long = type(getattr(sys, 'maxint', 1) + 1)
+    assert isinstance(m.int_cast(), int)
+    assert isinstance(m.long_cast(), int)
+    assert isinstance(m.longlong_cast(), must_be_long)