Avoid string copy if possible when passing a Python object to std::ostream

[jonathan-conder-sm]

Description

Changes operator<<(std::ostream &, const handle &) to delegate to operator<<(std::ostream &, std::string_view) if possible. If I understand the type caster for string_view correctly, I think this avoids an unnecessary copy.

The PYBIND11_HAS_STRING_VIEW macro is defined in cast.h, which is indirectly included by stl.h. If that's likely to change in future, it might be desirable to move the macro to internals.h or something.

Suggested changelog entry:

Use ``std::string_view`` if available to avoid a copy when passing an object to a ``std::ostream``.

[rwgk]

Thanks!