website: Remove reference to python3-six in the building gem5 doc

https://gem5-review.googlesource.com/c/public/gem5/+/57969

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: If2c7c92809a2194df630c0f6044ebf44ff5f30b6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/57930
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index a2af8e8..fc490d4 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -51,7 +51,7 @@
 libraries. It is a necessary dependency if you wish to use the SystemC
 implementation.
 
-### Setup on Ubuntu 18.04
+### Setup on Ubuntu 18.04 (gem5 >= v21.0)
 
 If compiling gem5 on Ubuntu 18.04, or related Linux distributions, you may
 install all these dependencies using APT:
@@ -59,10 +59,10 @@
 ```
 sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python3-dev python3-six python libboost-all-dev pkg-config
+    python3-dev python libboost-all-dev pkg-config
 ```
 
-### Setup on Ubuntu 20.04
+### Setup on Ubuntu 20.04 (gem5 >= v21.0)
 
 If compiling gem5 on Ubuntu 20.04, or related Linux distributions, you may
 install all these dependencies using API:
@@ -70,7 +70,7 @@
 ```
 sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python3-dev python3-six python-is-python3 libboost-all-dev pkg-config
+    python3-dev python-is-python3 libboost-all-dev pkg-config
 ```
 
 ### Docker