[LyX/master] Add new placeholder $${python} to configure

[Pavel Sanda]

On Sat, Jan 22, 2022 at 09:54:39PM +0000, José Abílio Matos wrote:
> The problem here is that the placeholder is not replaced before being used.
> In order to see what python is being used you can see:
> 
> Help->About LyX

python3 -tt

> In a hunch does the attached patch fixes the issue?
> The idea is to replace the placeholder when the converter is added/imported.

Strangely, I can't reproduce the problem anymore. I'll wait if I can catch it again.
Pavel