Use python3 packages for Ubuntu (#6443)

INSTALL.md

@@ -109,8 +109,8 @@ Installing prerequisites on Ubuntu or Debian:
 
 ```
 sudo apt-get install build-essential python3-dev libffi-dev \
-                     python-pip python-setuptools sqlite3 \
-                     libssl-dev python-virtualenv libjpeg-dev libxslt1-dev
+                     python3-pip python3-setuptools sqlite3 \
+                     libssl-dev python3-virtualenv libjpeg-dev libxslt1-dev
 ```
 
 #### ArchLinux

changelog.d/6443.doc

@@ -0,0 +1 @@
+Switch Ubuntu package install recommendation to use python3 packages in INSTALL.md.