xlrd is a library for reading data and formatting information from
Excel files in the historical .xls format.
Version 2 of xlrd no longer supports anything other than .xls files.
We were using it to also support reading .xlsm files (old Excel files
with macro support).
We could keep using the old version of this dependency, but hopefully
this niche version of an ancient file format is obscure enough that
no-one is using it, and we can drop support, keeping our dependencies
up to date.
There shouldn’t be a case where we see a `ValueError` on upload any
more. Our file handling should be robust enough to deal with whatever is
thrown at it.
This commit:
- adds test files with bad data (PNG files with their extensions changed to look
like spreadsheets)
- catches whatever exceptions are raised by trying to parse these files
- returns a helpful flash message to the user
Anything else should raise a `500`, eg if the file can’t be uploaded to S3.
