Port event_rpcgen.py and test/check-dumpevents.py to Python 3.

These scripts remain compatible with Python 2.

CMakeLists.txt

@@ -915,19 +915,18 @@ if (NOT EVENT__DISABLE_TESTS)
     # Generate Regress tests.
     #
     if (NOT EVENT__DISABLE_REGRESS)
-        # (We require python2 to generate the regress tests)
-        foreach (PY python2.6 python2.7 python2)
-            unset(FIND_PYTHON2 CACHE)
-            find_program(FIND_PYTHON2 ${PY})
-            if (FIND_PYTHON2)
-                set(PYTHON_EXECUTABLE "${PY}")
-                break()
-            endif()
-        endforeach()
-        find_package(PythonInterp)
+        # (We require python to generate the regress tests)
+        find_package(PythonInterp 3)
 
-        if (PYTHONINTERP_FOUND AND PYTHON_VERSION_STRING VERSION_LESS "3.0.0")
+        if (PYTHONINTERP_FOUND)
             set(__FOUND_USABLE_PYTHON 1)
+        else()
+            find_package(PythonInterp 2)
+            if (PYTHONINTERP_FOUND)
+                set(__FOUND_USABLE_PYTHON 1)
+            else()
+                message(ERROR "No suitable Python version found, bailing...")
+            endif()
         endif()
 
         if (__FOUND_USABLE_PYTHON)