fix(python): force Python 3.8 on the internal CI

The internal CI uses Python 3.6 by default, which is no longer
sufficient for our dependencies. Python 3.8 is available, just
not used by default, so force our scripts to use it.

This also forces us to use `virtualenv` over `venv`, as the latter
is not available internally.

Change-Id: Idb460ab05053c80700366f02c8c70b1c27578483
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/script/install_python_deps_tf.sh b/script/install_python_deps_tf.sh
index 62fd795..242458e 100644
--- a/script/install_python_deps_tf.sh
+++ b/script/install_python_deps_tf.sh
@@ -1,12 +1,16 @@
 #!/usr/bin/env bash
 
 #
-# Copyright (c) 2021 Arm Limited. All rights reserved.
+# Copyright (c) 2021-2023 Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
 
-python3 -m venv .venv
+if [ $(python3 -c 'import sys; print(sys.version_info[1])') -lt 7 ]; then
+    function python3() { python3.8 "${@}"; }
+fi
+
+python3 -m virtualenv .venv
 
 source .venv/bin/activate