- restore compatibility with Julia <1.9 by re-introducing the PyPlot
  dependency (will be ignored on newer Julia versions) and hard
  including the extension of weakdeps are not available
- move ransacdemo.jl into the extension module
- add to README.md how to get plotting back on new Julia
- remove the plotting.jl stub, where the functions that extensions need
  to implement were declared.

Based on
https://pkgdocs.julialang.org/v1/creating-packages/#Backwards-compatibility
this uses Requires.jl to bring the weak dependency feature to Julia <1.9

move plotting.jl back in... extensions cannot add new functions, so we
already need to define them in a stub that raises an error. We should
think about what to export here.

When using Requires and/or package extensions, the PyPlot dependency is
not needed anymore. I guess I just forgot to take it out..

By requiring a higher Julia version, the package extension mechanism
will be available and can be used always (no need for Requires.jl
anymore). This makes the implementation easier.
The `get_plot_backend()` function is still contained to avoid method
overwrite warnings. Usually, a package extension does type piracy and
dispatches on a new type. We only have the demo functions, which cannot
dispatch on the type.
This now can be extended to different plotting backends (Makie for
example), which in turn could be selected in the `get_plot_backend()`
function.