Python: Add exec as a shared built-in

This is _slightly_ wrong, since `exec` isn't a built-in function in
Python 2. It should be harmless, however, since `exec` is a keyword,
and so cannot be redefined anyway.

Author: tausbn

python/ql/src/semmle/python/ApiGraphs.qll

@@ -372,7 +372,9 @@ module API {
           "RuntimeWarning", "StandardError", "StopIteration", "SyntaxError", "SyntaxWarning",
           "SystemError", "SystemExit", "TabError", "TypeError", "UnboundLocalError",
           "UnicodeDecodeError", "UnicodeEncodeError", "UnicodeError", "UnicodeTranslateError",
-          "UnicodeWarning", "UserWarning", "ValueError", "Warning", "ZeroDivisionError"
+          "UnicodeWarning", "UserWarning", "ValueError", "Warning", "ZeroDivisionError",
+          // Added for compatibility
+          "exec"
         ]
       or
       // Built-in constants shared between Python 2 and 3