prereqs: Ensure we use python 3.9

The GDB dependency is now on python 3.9, not 3.8.

Change-Id: I4e3b066fcb06fb788250ef22a09e3978d9b834d2

install-prereqs.sh

diff --git a/install-prereqs.sh b/install-prereqs.sh
index 31951f5..038f7cc 100755
--- a/install-prereqs.sh
+++ b/install-prereqs.sh
@@ -55,7 +55,7 @@
     libglib2.0-dev-bin
     libgtk2.0-0
     libpixman-1-dev
-    libpython3.8
+    libpython3.9
     libsqlite3-dev
     libssl-dev
     libtinfo-dev