fix(docs): make generate_test_list backward compatible

The recently introduced script generate_test_list.py [1] relies on
features from Python 3.9. This breaks the build for users using earlier
python versions e.g.  python 3.8 deployed in Ubuntu 20.04 LTS. Bump the
minimum version requirements and make the script backward compatible
with older versions of Python.

[1] https://review.trustedfirmware.org/c/TF-A/tf-a-tests/+/21135

Change-Id: I07fd47af0309787b08cd0f77aaeb7d50b9cae144
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/docs/getting_started/requirements.rst b/docs/getting_started/requirements.rst
index f38964a..ca1a0c5 100644
--- a/docs/getting_started/requirements.rst
+++ b/docs/getting_started/requirements.rst
@@ -25,7 +25,7 @@
 
     sudo apt-get install device-tree-compiler build-essential git python3
 
-Note that at least Python 3.7 is required.
+Note that at least Python 3.8 is required.
 
 Download and install the GNU cross-toolchain from Arm. The TF-A Tests have
 been tested with version 12.2.Rel1 (gcc 12.2):