Python2 deprecated native method import #924

Environment

• Pythonnet version: latest
• Python version: 3.7.4
• Operating System: MacOS 10.14.6

Details

• Compiling under xcode for iOS, it throughs PyClass_New reference not found error.
  After diggin' around for a while, there is no PyClass_New definition in Python3 actually.
  Also there are other deprecated python3 APIs need to be nested in #if PYTHON2 condition.

  PyLong_GetMax(not exists)
  PyObject_GC_New(name error, should be _PyObject_GC_New)

[DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
 internal static extern IntPtr PyClass_New(IntPtr bases, IntPtr dict, IntPtr name);