Remove mentions of python 3.8/3.9 support from README

We have not supported Python 3.8 for about two months at this point
with our minimum supported version being 3.10. Remove mentions of
3.8/3.9 as being supported in the README.

README.md

@@ -53,7 +53,7 @@ values for the flag could be:
 Currently, the assumptions for the system are:
 
 *   Recent Ubuntu distro, e.g. 20.04
-*   python 3.8.x/3.9.x/3.10.x
+*   python 3.10.x/3.11.x
 *   for local training, which is currently the only supported mode, we recommend
     a high-performance workstation (e.g. 96 hardware threads).