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

[Pavel Sanda]

On Sun, Jan 23, 2022 at 04:06:07PM +0000, José Abílio Matos wrote:
> Could it be a cache interfering?
> Are the previewed files cached?

I don't think so. I also checked whether missing reconfigure could have been
the case, but it does not look like that either.
Last thing coming to my mind is that I was not working on master's HEAD but
few commits back when the $$(python) transition was not finished.

Who knows, I'll keep an eye on it.

Pavel