Update the QSymm test suite to specify shape and format to empty models
When running the test suite with the latest Qsymm I get the following DeprecationWarnings:
...
kwant/tests/test_qsymm.py::test_real_space_basis
kwant/tests/test_qsymm.py::test_real_space_basis
/home/jbw/.local/miniconda/envs/kwant-latest/lib/python3.6/site-packages/qsymm/model.py:225: DeprecationWarning: Provide `shape` and `format` when initializing empty Model.
warn('Provide `shape` and `format` when initializing empty Model.', DeprecationWarning)
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