This is partly motivated by the MaD tests which looks much better now in
my opinion.
I also wanted this for testing argument passing. In Python we're
adopting the same argument positions as Ruby has
[here](4f3751dfea/ruby/ql/lib/codeql/ruby/dataflow/internal/DataFlowDispatch.qll (L508-L540))
So it would be nice if `arg[keyword foo]=...` was allowed, without
having to transform the `toString()` result of an argument position into
something without a space.
c1e6996e99