Mark test_ctrl_c_handler as timing sensitive

Mark test_ctrl_c_handler as timing sensitive

psycopg> =================================== FAILURES ===================================
psycopg> _____________________________ test_ctrl_c_handler ______________________________
psycopg>         t0 = time.time()
psycopg>         proc = sp.Popen([sys.executable, "-s", "-c", script], creationflags=creationflags)
psycopg>         proc.communicate()
psycopg>         t = time.time() - t0
psycopg>         assert proc.returncode == 0
psycopg> >       assert 1 < t < 2
psycopg> E       assert 3.7456982135772705 < 2
psycopg>
psycopg> tests/test_concurrency.py:293: AssertionError