Tests choke on github actions pypy runner

Author: michalsta

tests/Python/test_IsoSpecPy.py

@@ -27,6 +27,9 @@ def sprint(s):
 # - but is still WAAAY too strict. Often we can justifiably get different configurations, or different counts of configurations...
 
 molecules = "H2O1 C100 P1 P100 C1 H10C10O10N10S5 Se1 Se10 Sn1 Sn4 Sn4C1 C2H6O1 C1000 C520H817N139O147S8 C1H1O2N2Se1Sn1P1 P1C1Sn1 Se5 Sn5 Se50 Sn15 Se2Sn2C2O2N2S2B2He2U2Na2Cl2".split()
+import platform
+if platform.python_implementation() != "CPython":
+    molecules = molecules[:5]  # limit test size on non-CPython implementations
 
 parameters = list(map(float, "0.0 0.1 0.5 0.01 0.9 0.99 0.01 0.0001 0.999 0.362 0.852348".split()))