Update bindep.txt for LaTeX pict2e.sty optional dependency

[jfbu]

Refs: #10648

When building PDF via LaTeX, the pict2e LaTeX package will be loaded:

• if in conf.py the 'sphinxsetup' key of latex_elements has been used with one of the radii setting CSS-named-alike options added at LaTeX: box padding (t,r,b,l), borders (t,r,b,l), shadows (x,y) or radii #10648
• and if it actually exists on user system,

So it is not a binding dependency as in case of its absence the make latexpdf falls back to rendering with straight corners.

Nevertheless adding it here.

[jfbu]

Thanks for review @AA-Turner! now merging...