Mismatched monthly downloads, total not updated(?), necessary clean-ups

[h-vetinari]

Since .conda artefacts were enabled, our main status page has shown these numbers

They don't seem to move though, which is impossible (esp. for the total). Now, after ContinuumIO/anaconda-package-data#45 was fixed, I reran the total downloads.ipynb notebooks, and I'm getting:

In particular, the last few monthly total download numbers (in millions) have been

2024-08-01    810.585658
2024-09-01    763.219017
2024-10-01    837.617099
2024-11-01    851.592709

i.e. quite a bit removed from the claimed 2.15 billion.

Furthermore, there's a fair amount of clean-ups to do here:

• update python
  

  by-the-numbers/environment.yml

  Line 5 in 8ab2886

  - python=3.8

• unpin pandas
  

  by-the-numbers/conda-requirements.txt

  Line 4 in 8ab2886

  pandas<2

• remove cell outputs from what's committed (e.g. using nbstripout git hooks)
• ensure all the notebooks are still runnable, produce useful output (alternatively, remove)