Bump protobuf requirement to 3.12.x (#3132)

Fixes #3110.

Author: balopat

cirq/google/arg_func_langs_test.py

@@ -105,7 +105,12 @@ def test_serialize_sympy_constants():
                                        including_default_value_fields=True,
                                        preserving_proto_field_name=True,
                                        use_integers_for_enums=True)
-    assert packed == {'arg_value': {'float_value': float(np.float32(sympy.pi))}}
+    # protobuf 3.12+ truncates floats to 4 bytes
+    assert packed == {
+        'arg_value': {
+            'float_value': float(str(np.float32(sympy.pi)))
+        }
+    }
 
 
 def test_unsupported_function_language():

requirements.txt

@@ -7,7 +7,7 @@ matplotlib~=3.0
 networkx~=2.4
 numpy~=1.16
 pandas
-protobuf==3.8.0
+protobuf~=3.12.0
 requests~=2.18
 sortedcontainers~=2.0
 scipy