Disable baacketed-paste mode in bash 5.1 and newer (issue #669)

The bash versions starting with 5.1 have bracketed paste mode enabled
by default, leading to test failures in replwrap.bash(), e.g.:

self = <tests.test_replwrap.REPLWrapTestCase testMethod=test_multiline>

    def test_multiline(self):
        bash = replwrap.bash()
        res = bash.run_command("echo '1 2\n3 4'")
>       self.assertEqual(res.strip().splitlines(), ['1 2', '3 4'])
E       AssertionError: Lists differ: ['\x1b[?2004l', '\x1b[?2004h\x1b[?2004l',
'1 2', '3 4', '\x1b[?2004h'] != ['1 2', '3 4']
E
E       First differing element 0:
E       '\x1b[?2004l'
E       '1 2'
E
E       First list contains 3 additional elements.
E       First extra element 2:
E       '1 2'
E
E       - ['\x1b[?2004l', '\x1b[?2004h\x1b[?2004l', '1 2', '3 4', '\x1b[?2004h']
E       + ['1 2', '3 4']

With bracketed test mode disabled, the tests that were affected by the
inserted escape sequences succeed.