[JOSS]: sympy requirement for some python tests not mentioned in INSTALL.md

[wence-]

The requirement of some tests on sympy is not mentioned in the install instructions, and so collection of those tests fails.

Steps to reproduce

$ git clone git@github.com:fenics/basix.git
$ python3 -m venv basix-test
$ . basix-test/bin/activate
$ cd basix
$ pip install .
$ pip install numpy pytest
$ pytest --collect-only test/
[...]
test/test_rt.py:8: in <module>
    import sympy
E   ModuleNotFoundError: No module named 'sympy'
========================================================= short test summary info =========================================================
ERROR test/test_lagrange.py
ERROR test/test_nedelec.py
ERROR test/test_nedelec_second_kind.py
ERROR test/test_orthonormal_basis_symbolic.py
ERROR test/test_quadrature.py
ERROR test/test_rt.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 6 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
================================================= 3101 tests collected, 6 errors in 7.14s =================================================

After installing sympy the tests pass.