scripts: Add simpleeval to python-requirements.txt

simpleeval was removed from the camkes-deps requirement since 0.7.2, but
our sel4/capdl fork still uses it. Add the module explictly to the list.

Change-Id: I2577e5ebfebd13caacc7014d20c4e251eb11e1be

python-requirements.txt

diff --git a/python-requirements.txt b/python-requirements.txt
index 48a093d..343ffef 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -12,5 +12,6 @@
 robotframework==4.0.1
 sel4-deps
 setuptools
+simpleeval
 tempita
 wget