scons: Raise an exception when scons is run a Python2 environment

As gem5 has started to use Python2 incompatible features, compiling
gem5 in a Python2 environment results in an error.

This commit addresses this issue by raising an Exception when scons
is run in a Python2 environment, and adding a few pointers on how to
install Python3 and on how to use scons in a Python3 environment. The
solution works in a system where both Python2 and Python3 are
installed.

JIRA: https://gem5.atlassian.net/browse/GEM5-797

Change-Id: I98d4a39f586f39d9253ab2517b77e86c5ed19466
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36157
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/site_init.py

@@ -51,14 +51,37 @@ For more details, see:
 """)
     raise
 
-# pybind11 requires python 2.7
+
+# Check for the python version. Python 2 is no longer supported.
 try:
-    EnsurePythonVersion(2, 7)
+    EnsurePythonVersion(3, 0)
 except SystemExit as e:
-    print ("""
-You can use a non-default installation of the Python interpreter by
-rearranging your PATH so that scons finds the non-default 'python' and
-'python-config' first.
+    print("""\033[93m
+Python 3 is now required.
+
+The following are steps to compile gem5 in Python 3 environment,
+
+*Step 1*: ensure Python 3 is installed. On Ubuntu like systems, you can try \
+this command:
+
+    sudo apt-get install python3 python3-six python-is-python3 python3-pydot
+
+To run Python 3 from a container, you can try the Docker files in \
+util/dockerfiles folder.
+
+*Step 2*: ensure that scons is run in the Python 3 environment. If scons \
+isn't run automatically with Python 3, you can force it by replacing `scons` \
+by the following phrase,
+
+    /usr/bin/env python3 $(which scons)
+
+For example, the following command will let scons compile gem5/X86 in the \
+Python 3 environment,
+
+   /usr/bin/env python3 $(which scons) build/X86/gem5.opt
+
+(Optional) For convenience reasons, you can set up an alias for the Python3 \
+scons phrase in your environment. \033[0m
 """)
     raise