Failed test with Python 3.8

[jorti]

This test is failing with Python 3.8

=================================== FAILURES ===================================
____________________________ TestDebug.test_rebulk _____________________________

self = <rebulk.test.test_debug.TestDebug object at 0x7f5ead51dfa0>

    def test_rebulk(self):
        """
        This test fails on travis CI, can't find out why there's 1 line offset ...
        """
        assert self.rebulk._patterns[0].defined_at.lineno in [26, 27]
        assert self.rebulk._patterns[0].defined_at.name == 'rebulk.test.test_debug'
        assert self.rebulk._patterns[0].defined_at.filename.endswith('test_debug.py')
    
        assert str(self.rebulk._patterns[0].defined_at) in ['test_debug.py#L26', 'test_debug.py#L27']
    
>       assert self.rebulk._patterns[1].defined_at.lineno in [27, 28]
E       AssertionError: assert 26 in [27, 28]
E        +  where 26 = test_debug.py#L26.lineno
E        +    where test_debug.py#L26 = <StringPattern@test_debug.py#L26:('is',)>.defined_at

rebulk/test/test_debug.py:73: AssertionError
===================== 1 failed, 162 passed in 1.38 seconds =====================