Better workflow for drag & dropping excel files into Iodide

[wlach]

Dragging and dropping an excel file into an iodide notebook currently works (e.g. this notebook was created that way: https://alpha.iodide.io/notebooks/3662/) however there are no clear instructions for what to do with the result. For this particular case, we should probably pre-populate a notebook with the right incantations for loading it with pandas.

First step is to figure out how to load excel data into pandas at all :) /cc @deborahanneharris

[wlach]

I finally got it working yesterday after a bit of flailing around: https://definitely-staging.iodide.io/notebooks/221/

Notes:

• when you create a file by dragging, it always assumes the type is text (even when it's binary), filed Binary dragged & dropped files should not be fetched as text by default #2456 for this
• there are several non-trivial transformations that need to be made to the blob that was fetched before it can by input into pandas. Adding a "bytes" type should resolve this, see proposal: binary fetch type #1786
• currently need to manually install xlrd package via micropip, otherwise pandas will fail to load the file. Should be fixed by: Add xlrd python package (fixes #564) pyodide/pyodide#566

My dream workflow would be that dragging & dropping an excel file would generate something like the above notebook (without all the hacks required by the above, of course). /cc @hamilton @openjck