update reticulate dependency version

[goldingn]

The versions of greta on main and the new CRAN release on address-cran-patch-i792 have a stated dependency on reticulate >=1.19.0 (here). But they also use reticulate::py_require() in greta:::.onLoad (here). That reticulate function appears to have been introduced in reticulate version 1.43.0. This causes building/installation to fail if a version of reticulate >=1.19.0 but <1.43.0 is installed.

The fix is to update this line to reticulate >=1.43.0