Update requirements.txt

Author: pancetta

requirements.txt

@@ -3,7 +3,7 @@ numpy>=1.15.4
 scipy>=0.17.1
 dill>=0.2.6
 # These packages are used in examples or for testing, they can be omitted if necessary
-matplotlib>=3.0
+matplotlib>=3.0,<=3.5.3
 numba>=0.35
 sympy>=1.0
 # These packages are required for some of the more involved examples (and data types etc.), can be omitted