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

[guitargeek]

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:

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]);
                                        ^
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Reproducer:

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)

[pcanal]

LGTM. Consider adding a test though.