Unblock test that requires sympy-1.12 (#6096)

The test is not expected to fail anymore for Python 3.8 and later.
The test fails with Python 3.7 which is not supported by sympy 1.12.

Fix #6092

cirq-core/cirq/study/resolver_test.py

@@ -13,7 +13,9 @@
 # limitations under the License.
 
 """Tests for parameter resolvers."""
+
 import fractions
+import sys
 
 import numpy as np
 import pytest
@@ -241,7 +243,10 @@ def _resolved_value_(self):
     assert r.value_of(b) == 'Baz'
 
 
-@pytest.mark.xfail(reason='this test requires sympy 1.12', strict=True)
+# sympy 1.12 does not support Python 3.7
+@pytest.mark.xfail(
+    condition=sys.version_info < (3, 8, 0), reason='this test requires sympy 1.12', strict=True
+)
 def test_custom_value_not_implemented():
     class Bar:
         def _resolved_value_(self):