Prevent unwanted simplification of derivatives after SymPy upgrade

The `simplify` method changed in SymPy version 1.5 due to commit
sympy/sympy@0fdc617
to evaluate unevaluated derivatives by default. This can trivially be
disabled which is what this commit does.
Add a test to catch this in future.

checker/maths.py

@@ -180,7 +180,7 @@ def symbolic_equality(test_expr, target_expr):
     # it doesn't seem like much of an issue. Removing 'sympy.posify()' below will
     # stop this.
     try:
-        if sympy.simplify(sympy.posify(test_expr - target_expr)[0]) == 0:
+        if sympy.simplify(sympy.posify(test_expr - target_expr)[0], doit=False) == 0:
             print("Symbolic match.")
             print("INFO: Adding known pair ({0}, {1})".format(target_expr, test_expr))
             KNOWN_PAIRS[(target_expr, test_expr)] = EqualityType.SYMBOLIC

checker/tests/test_maths.py

@@ -498,6 +498,20 @@ def test_derivatives(self):
         self.assertTrue(response["equality_type"] == "symbolic", 'For these expressions, expected "equality_type" to be "symbolic", got "{}"!'.format(response["equality_type"]))
         print("   PASS   ".center(75, "#"))
 
+    def test_derivatives_not_removed(self):
+        print("\n\n\n" + " Test Derivatives Not Evaluated ".center(75, "#"))
+        test_str = "0"
+        target_str = "Derivative(y, x)"
+        response = api.check(test_str, target_str, check_symbols=False)
+
+        self.assertTrue("error" not in response, 'Unexpected "error" in response!')
+        self.assertTrue("equal" in response, 'Key "equal" not in response!')
+        self.assertTrue(response["equal"] == "false", 'Expected "equal" to be "false", got "{}"!'.format(response["equal"]))
+        self.assertTrue("equality_type" in response, 'Key "equality_type" not in response!')
+        self.assertTrue(response["equality_type"] in EQUALITY_TYPES, 'Unexpected "equality_type": "{}"!'.format(response["equality_type"]))
+        self.assertTrue(response["equality_type"] == "numeric", 'For these expressions, expected "equality_type" to be "numeric", got "{}"!'.format(response["equality_type"]))
+        print("   PASS   ".center(75, "#"))
+
     def test_differential_equations(self):
         print("\n\n\n" + " Test Differential Equations ".center(75, "#"))
         api.SIMPLIFY_DERIVATIVES = True