feat: any Mapping assigned to a WritableDirectory is interpreted as a TTree or failure, no fall-through.

[jpivarski]

This is a change in policy. Now when anyone does

with uproot.recreate("file.root") as file:
    file["something"] = {"key": value}

it will always attempt to interpret the {"key": value} Mapping (any kind of Mapping) as a TTree. If any value cannot be interpreted as an array, it raises an error message that highlights the "key" name.

The old behavior was to give up on interpreting {"key": value} as a TTree, but keep open the possibility that it might be some other kind of writable data. In practice, there are no other types of writable data associated with Mapping, so the error message would be "unrecognized type cannot be written to a ROOT file: __dict__'".

What's worse, this error message depended on the state of the environment: if awkward is not installed, then Uproot can't check non-NumPy value to see if it might be interpretable as an Awkward Array (with ak.from_iter), so it failed with the above complaint about not recognizing the dict type, instead of saying that awkward needs to be pip-installed.

[agoose77]

In general, I'm all for tightening our input validation. This seems like a good policy change.