[core] Avoid errors when using std::span backport from Python

The `vector<T> const&` constructor needs to be disabled if T is not a
const type, because we don't want to create `span<T>` from `vector<T>
const&`. The explicit disabling via SFINAE is necessary to this overload
is not accidentally taken by cppyy, resulting in the failure of
constructor wrapper code compilation.

Avoids errors like these when compiling ROOT with C++17:

```txt
In module 'Core':
/home/rembserj/code/root/root_install/include/ROOT/span.hxx:208:27: error: cannot initialize a member subobject of type 'pointer' (aka 'double *') with an rvalue of type 'const double *'
     : length_(v.size()), data_(v.empty() ? nullptr : v.data())
                          ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
input_line_45:7:41: note: in instantiation of member function 'std::span<double>::span' requested here
      (*(std::span<double>**)ret) = new std::span<double>((const std::vector<double, std::allocator<double> >&)*(const std::vector<double, std::allocator<double> >*)args[0]);
                                        ^
0
1
2
3
```

Reproducer:
```Python
import ROOT

ROOT.gInterpreter.Declare("""
void calc_cumsum(std::size_t n, std::span<double> v)
{
    for (int i = 0; i < n; i += 1) {
        std::cout << v[i] << std::endl;
    }
}
""")

v = ROOT.std.vector["double"](list(range(4)))

ROOT.calc_cumsum(len(v), v)
```

Author: guitargeek

core/foundation/inc/ROOT/span.hxx

@@ -204,6 +204,13 @@ public:
     static_assert(N > 0, "Zero-length array is not permitted in ISO C++.");
   }
 
+  // Note:
+  // This constructor needs to be disabled if T is not a const type, because we
+  // don't want to create span<T> from vector<T> const&.
+  // The explicit disabling via SFINAE is necessary to this overload is not
+  // accidentally taken by cppyy, resulting in the failure of constructor
+  // wrapper code compilation.
+  template <class U = T, class = typename std::enable_if<std::is_const<U>::value>::type>
   /*implicit*/ span(std::vector<typename std::remove_cv<T>::type> const& v) noexcept
      : length_(v.size()), data_(v.empty() ? nullptr : v.data())
   {}