Add optional buffer size to pythonbuf::d_buffer constructor (#1687)

In some cases the user of pythonbuf needs to allocate the internal
buffer to a specific size e.g. for performance or to enable synchronous
writes to the buffer.
By changing `pythonbuf::d_buffer` to be dynamically allocated we can now
enable these use-cases while still providing the default behavior of
allocating a 1024 byte internal buffer (through a default parameter).