Add `pouplar` -> `popular`

[mara004]

See pypdfium2-team/pypdfium2#436
Not noticed by codespell yet, it seems.

[Marcobanderas]

#3940

[Marcobanderas]

https://github.com/pypdfium2-team/pypdfium2/pull/436https://canva.link/svrn6dt6c6uv8jh

[mara004]

Reported @Marcobanderas to GH for potential Phishing.

[peternewman]

Also poplar (the tree).