Fix issues found by flake8-bugbear:
* Unused loop variables
* assert statements in non-test code
* mixing test code with production code
* mark one excessive test which takes ~10min to run
as 'slow'
In the BASIC version the calculation is on line 455:
`455 IF D<10*P THEN 510`
Which skips over the not enough people message.
In the Python version the logic is reversed, and we check
to see if there is too few people and then run the message:
`elif D >= 10 * P` (in the current code).
However, this means that the case where you want to plant the
maximum number of acres won't work.
e.g. You have 100 people (P) and want to plant 1000 acres (D).
`1000 >= 10 * 100`
`1000 >= 1000`
Which triggers the "not enough people code".
Maybe this is a bug in the original program.
This fixes an error where the harvest was only added
to the total bushels if rats ate some.
Note in the BASIC file, we check to see if rats eat
the grain in line 522 and if not, go to line 530,
which is where the `S=S-E+H` calculation is done.
