Removing state pollution in parser_cache

[sturmianseq]

This PR aims to improve test reliability of test test_permission_error by removing state pollution in parser_cache by calling method clear

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

        monkeypatch.setattr(cache, '_save_to_file_system', save)
        with pytest.warns(Warning):
>           parse(path=__file__, cache=True, diff_cache=True)
E           Failed: DID NOT WARN. No warnings of type (<class 'Warning'>,) was emitted. The list of emitted warnings is: [].

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.

[isidentical]

This looks OK, though I'd prefer either a try/finally on the parse call or an explicit fixture that does something like this;

@pytest.fixture
def clear_cache():
    yield
    parser_cache.clear()

to make sure that we clear it in either way.

[davidhalter]

I agree with @isidentical

[sturmianseq]

Thanks for your comment. The PR is updated with try & finally blocks applied.