Clean up lesson with precise language and formatting

[deppen8]

• Make references more explicitly to pandas rather than "Python"
• Add lots of code style formatting where appropriate
• Replace references to isnull() with isna(). These methods are exactly the same for pandas, but isna() is consistent with fillna, dropna, etc. which are also used here.
• Replaced a reference to survey_df.weight with survey_df['weight'], the preferred notation style for pandas

[maxim-belkin]

Thanks again, @deppen8! I agree with most of the changes but I'm not sure about using pandas vs pandas. I think the fastest way forward would be to address this question in a separate PR.

[tobyhodges]

I'm helping the current lesson Maintainers process outstanding pull requests on this repository, in preparation for transition to the new lesson infrastructure.

Thank you for this contribution, @deppen8. Based on Maxim's comments above, it seems this PR is almost ready to merge. The only comment to be addressed is how to style the word "pandas" throughout. Maxim suggested reverting these changes here and opening a separate PR but (and especially because we need to close all PRs before the end of this month) I propose instead that we make a change in this branch.

The pandas website and documentation seem to take the approach of referring to the library as "pandas" (always with a lowercase "p") and only writing pandas when referring to the imported modules themselves. I think we should follow the same principle here, in which case this PR should be updated.

@deppen8 if you have the time and inclination to make these changes before the end of April 2023, please go ahead and do so. If not, I will make the changes myself and merge the PR at the end of the month, before the infrastructure transition takes place at the beginning of May.

[tobyhodges]

Superseded by #541