[FEAT] Get a read-only view of the data in py::bytes

Looking at the pybind11::bytes class (in pytypes.h), it is not obvious how to get the data out without first making an unnecessary std::string copy of it.

This function is not terribly useful:

operator std::string() const {
    char *buffer;
    ssize_t length;
    if (PYBIND11_BYTES_AS_STRING_AND_SIZE(m_ptr, &buffer, &length))
        pybind11_fail("Unable to extract bytes contents!");
    return std::string(buffer, (size_t) length);
}

More useful:

operator byte_view() const {
    char *buffer;
    ssize_t length;
    if (PYBIND11_BYTES_AS_STRING_AND_SIZE(m_ptr, &buffer, &length))
        pybind11_fail("Unable to extract contents of bytes object!");
    return { as_unsigned(buffer), as_unsigned(length) };
}

// Boiler plate
using byte_view = std::basic_string_view<uint8_t>;
size_t as_unsigned(ssize_t val) { return static_cast<size_t>(val); }
const uint8_t *as_unsigned(const char *s) { return reinterpret_cast<const uint8_t *>(s); }

Start in pytypes.h by reading pybind11::bytes and its existing operator std::string() implementation, then assess the proposed read-only byte_view interface and its ownership and type-conversion implications. Done means providing a documented way to view the bytes data without an unnecessary std::string copy, with behavior consistent with the existing error handling.