quiet clang warning by adding default move ctor

[tdp2110]

clang-8 warns:

../ThirdParty/build/install/include/pybind11/iostream.h:124:5: warning: explicitly defaulted move constructor is implicitly deleted [-Wdefaulted-function-deleted]
    scoped_ostream_redirect(scoped_ostream_redirect&& other) = default;
    ^
../ThirdParty/build/install/include/pybind11/iostream.h:110:23: note: move constructor of 'scoped_ostream_redirect' is implicitly deleted because field 'buffer' has a deleted move constructor
    detail::pythonbuf buffer;
                      ^
../ThirdParty/build/install/include/pybind11/iostream.h:30:29: note: copy constructor of 'pythonbuf' is implicitly deleted because field 'd_buffer' has a deleted copy constructor
    std::unique_ptr<char[]> d_buffer;
                            ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unique_ptr.h:685:7: note: 'unique_ptr' has been explicitly marked deleted here
      unique_ptr(const unique_ptr&) = delete;
      ^
1 warning generated.

pythonbuf holds a unique_ptr (hence has no copy ctor), and defines a destructor, and hence should have a deleted move ctor (https://en.cppreference.com/w/cpp/language/move_constructor)

[wjakob]

LGTM, thanks.