fix multiple warnings

• Use the undeprecaded sympy objects and imports
• Avoid potentially creating an array of arrays with different shape
• Properly use pytest.warns
• Update dependencies to match upcoming Kwant
• Silence the unfixable warnings from old scipy and sympy in pytest.

environment-latest.yml

@@ -2,7 +2,7 @@ name: qsymm-latest
 channels:
     - conda-forge
 dependencies:
-    - python=3.8
+    - python=3.9
     - numpy
     - scipy
     - sympy