requirements sympy >= 1.7

[jvanhoefer]

just a minor thing, but the newest AMICI version seems to need simply version 1.7, with 1.6.1 I got an ImportError for import amici in

AMICI/python/amici/ode_export.py

Line 34 in 275e599

from sympy.printing.codeprinter import cxxcode

However, sympy >= 1.7 seems to be not in the requirements (at least pip didn't automatically upgrade, when installing the newest AMICI...)

[lcontento]

In SymPy 1.6 cxxcode is in sympy.printing.cxxcode. But it should be sufficient to import it from sympy.printing which should work with both versions. Will make a small PR.