Tools: Constrain the cffi package for older pythons

requirements.txt

@@ -9,6 +9,8 @@ click>=7.0
 pyserial>=3.0
 future>=0.15.2
 cryptography>=2.1.4,<35
+# cffi 1.15 supports Python 3.6+ only
+cffi<1.15;python_version<'3.6'
 pyparsing>=2.0.3,<2.4.0
 pyelftools>=0.22