Replace call to execfile(...) by call to exec(compile(open(...)))

This is done in order to be compatible with Python3

python/mach_bootstrap.py

@@ -166,7 +166,7 @@ def _activate_virtualenv(topdir, is_firefox):
         # We want to upgrade pip when virtualenv created for the first time
         need_pip_upgrade = True
 
-    execfile(activate_path, dict(__file__=activate_path))
+    exec(compile(open(activate_path).read(), activate_path, 'exec'), dict(__file__=activate_path))
 
     python = _get_exec_path(PYTHON_NAMES, is_valid_path=check_exec_path)
     if not python:

python/servo/testing_commands.py

@@ -350,7 +350,7 @@ def test_webidl(self, quiet, tests):
 
         run_file = path.abspath(path.join(test_file_dir, "runtests.py"))
         run_globals = {"__file__": run_file}
-        execfile(run_file, run_globals)
+        exec(compile(open(run_file).read(), run_file, 'exec'), run_globals)
 
         verbose = not quiet
         return run_globals["run_tests"](tests, verbose)
@@ -442,7 +442,7 @@ def wptrunner(self, run_file, **kwargs):
             kwargs["binary_args"] = binary_args
 
         run_globals = {"__file__": run_file}
-        execfile(run_file, run_globals)
+        exec(compile(open(run_file).read(), run_file, 'exec'), run_globals)
         return run_globals["run_tests"](**kwargs)
 
     @Command('update-manifest',
@@ -473,7 +473,7 @@ def update_wpt(self, **kwargs):
             return 1
 
         run_globals = {"__file__": run_file}
-        execfile(run_file, run_globals)
+        exec(compile(open(run_file).read(), run_file, 'exec'), run_globals)
         return run_globals["update_tests"](**kwargs)
 
     @Command('filter-intermittents',
@@ -913,7 +913,7 @@ def update_net_cookies(self):
                                           "components", "net", "tests",
                                           "cookie_http_state_utils.py"))
         run_globals = {"__file__": run_file}
-        execfile(run_file, run_globals)
+        exec(compile(open(run_file).read(), run_file, 'exec'), run_globals)
         return run_globals["update_test_file"](cache_dir)
 
     @Command('update-webgl',
@@ -934,7 +934,7 @@ def update_webgl(self, version=None):
             shutil.rmtree(dest_folder)
 
         run_globals = {"__file__": run_file}
-        execfile(run_file, run_globals)
+        exec(compile(open(run_file).read(), run_file, 'exec'), run_globals)
         return run_globals["update_conformance"](version, dest_folder, None, patches_dir)
 
     @Command('smoketest',