skip qsymm tests if sympy and qsymm are not installed
This is a partial application of the changeset proposed in !274/, which removes the need for a 'conftest.py', instead using 'pytest.importorskip'. We apply this change only to 'qsymm', as applying this change elsewhere requires rethinking where tests are put in the package structure of Kwant. This minimal change allows us to build packages for Conda and Debian, which is necessary for releasing Kwant 1.4.
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