Avoiding order dependency by altering pytest.fixture scope to function

[sturmianseq]

This PR aims to improve test reliability of tests in test_multiindex.py by avoiding order dependency by altering pytest.fixture scope to function.

The test can fail in this way by running pip3 install pytest-repeat; python3 -m pytest multiindex/tests/test_multiindex_hashed_non_unique.py --count=2
:

    def test_remove(mi):
        mi.remove('first_name', 'George')
>       assert len(mi.get('first_name', 'George')) == 1
E       AssertionError: assert 0 == 1
E        +  where 0 = len([])
E        +    where [] = <bound method MultiIndexContainer.get of <multiindex.multiindex.MultiIndexContainer object at 0x7f3aef5d51c0>>('first_name', 'George')
E        +      where <bound method MultiIndexContainer.get of <multiindex.multiindex.MultiIndexContainer object at 0x7f3aef5d51c0>> = <multiindex.multiindex.MultiIndexContainer object at 0x7f3aef5d51c0>.get

It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state pollution.