How to control which Python interpreter Postgres uses?

I have started writing PL/Python scripts on a CentOS 7 box running PG 11. The issue I have is that by default my PG instance wants to use python3.6 instead of version 3.9 which I would prefer. I can see this by running a simple PL/Python script that reports the version of Python being used.

Postgres was installed using yum:

```
$ sudo yum list installed | grep postgres
postgresql11.x86_64 11.13-1PGDG.rhel7 @pgdg11
postgresql11-libs.x86_64 11.13-1PGDG.rhel7 @pgdg11
postgresql11-plpython3.x86_64 11.13-1PGDG.rhel7 @pgdg11
postgresql11-server.x86_64 11.13-1PGDG.rhel7 @pgdg11
```

I re-created the sym link for /usr/bin/python3 to point to /usr/local/bin/python3.9 however there is no change. It appears that the installation is hard-coded to point to /usr/bin/python3.6 as the Python interpreter.

How can I adjust my postgres settings to tell it which Python interpreter binary to use?

Thanks!

-nine