added vignette on Python and flair

[ttimbers]

@kbodwin - I have created this PR to add a vignette on Python and flair. However, it depends on Python, {reticulate}, and pandas. So if you incorporate this PR, you will need to install those things to rebuild your site with {pkgdown}...

Also, I am a little stuck on using pkgdown::build_site as although I can use the knit button to render the vignette, when I run pkgdown::build_site it cannot find Python on my machine. Which is bizarre as I said above I can render it via the knit button which uses both Python and reticulate. I am going to open an issue in the {pkgdown} repo to find out how to deal with this... Fixed!

[ttimbers]

I think the build is failing on TravisCI because of the missing dependencies I mentioned above. Two possible solutions:

1. Add these dependencies to the .travis.yml file

2. Add the vignette to the .Rbuildignore and not rebuild it each time you run R CMD build

I am not well versed in TravisCI (I have more experience with GitHub actions...) so I am not sure how much help I will be with suggestion # 1 (I know it's possible, I could probably do it, but is it worth the effort for the vignette is the question).