Raise minimum sympy version to 1.7.1 (#1368)
Raise minimum sympy version to 1.7.1 (#1368)

Closes #1367
FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI