fix python version check (#705)

Commit 11a337f added major and minor python version
checking to cast.h but does not use the macros defined
via the Python.h inclusion. This may be due to an
intention to use the variables defined by the cmake
module FindPythonInterpreter, but nothing in the
pybind11 repo does anything to convert the cmake
variables to preprocessor defines.

Author: eirrgang

include/pybind11/cast.h

@@ -646,14 +646,14 @@ struct type_caster<std::basic_string<CharT, Traits, Allocator>, enable_if_t<is_s
     using StringType = std::basic_string<CharT, Traits, Allocator>;
 
     bool load(handle src, bool) {
-#if PY_VERSION_MAJOR < 3
+#if PY_MAJOR_VERSION < 3
         object temp;
 #endif
         handle load_src = src;
         if (!src) {
             return false;
         } else if (!PyUnicode_Check(load_src.ptr())) {
-#if PY_VERSION_MAJOR >= 3
+#if PY_MAJOR_VERSION >= 3
             return false;
             // The below is a guaranteed failure in Python 3 when PyUnicode_Check returns false
 #else