fix: __index__ on Enum should always be present. (#3700)

* chore: minor odd py version cleanup

* Update include/pybind11/pybind11.h

* fix: always make __index__ available

Author: henryiii

include/pybind11/detail/common.h

@@ -153,9 +153,6 @@
 
 /// Include Python header, disable linking to pythonX_d.lib on Windows in debug mode
 #if defined(_MSC_VER)
-#  if (PY_MAJOR_VERSION == 3 && PY_MINOR_VERSION < 4)
-#    define HAVE_ROUND 1
-#  endif
 #  pragma warning(push)
 // C4505: 'PySlice_GetIndicesEx': unreferenced local function has been removed (PyPy only)
 #  pragma warning(disable: 4505)

include/pybind11/pybind11.h

@@ -1911,13 +1911,10 @@ template <typename Type> class enum_ : public class_<Type> {
         def(init([](Scalar i) { return static_cast<Type>(i); }), arg("value"));
         def_property_readonly("value", [](Type value) { return (Scalar) value; });
         def("__int__", [](Type value) { return (Scalar) value; });
+        def("__index__", [](Type value) { return (Scalar) value; });
         #if PY_MAJOR_VERSION < 3
             def("__long__", [](Type value) { return (Scalar) value; });
         #endif
-        #if PY_MAJOR_VERSION > 3 || (PY_MAJOR_VERSION == 3 && PY_MINOR_VERSION >= 8)
-            def("__index__", [](Type value) { return (Scalar) value; });
-        #endif
-
         attr("__setstate__") = cpp_function(
             [](detail::value_and_holder &v_h, Scalar arg) {
                 detail::initimpl::setstate<Base>(v_h, static_cast<Type>(arg),