Update test requirements files and docs

Author: janbridley

README.rst

@@ -75,7 +75,8 @@ additional dependencies are required to run the tests and build the docs.
 .. code:: bash
 
     pip install .            # Install with no additional dependencies
-    pip install .[tests]     # Install with dependencies required to run tests
+    pip install .[sympy]     # Install with sympy for symbolic unit cell math
+    pip install .[tests]     # Install with dependencies required to run tests (including sympy)
     pip install .[tests,doc] # Install with dependencies required to run tests and make docs
 
 Dependencies

tests/requirements.in

@@ -3,3 +3,4 @@ gemmi
 pycifrw<5.0.0
 pytest
 pytest-doctestplus
+sympy