migration to package extensions #414
                
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Closes #405.
We can hopefully remove all these strong dependencies.
Markdown: needed for interactive help (probably always loaded by Julia anyway)juliacall.PkgREPL: it is only imported when needed (i.e. when juliacall is loaded in an interactive Python session)Serialization: this is needed by__reduce__in jlwrap anyway (unless we load it on demand??)