Fail gracefully when parsing takes too long

[abulte]

Cf #80

[abulte]

File included for reproduction:
data.gouv-mars2022.xlsx

After a little research:

• we can't rely on asyncio.wait_for to wrap the parsing with a timeout (probably because the parsing blocks everything)
• the underlying library of agate-excel (openpyxl) has not trouble opening the file, hanging happens when converting to agate.Table
• we could try to wrap the synchronous part of parsing in a timeout, but this would involve threads in an async env (mayhem)
• maybe limit based on number of columns detected?