Merge pull request #1402 from davidzchen/py2and3

Add missing PY2AND3 srcs_versions attributes to Python Bazel build targets

BUILD

@@ -560,6 +560,7 @@ py_library(
             "python/google/protobuf/internal/test_util.py",
         ],
     ),
+    srcs_version = "PY2AND3",
     imports = ["python"],
 )
 
@@ -644,6 +645,7 @@ py_proto_library(
     include = "src",
     default_runtime = "",
     protoc = ":protoc",
+    srcs_version = "PY2AND3",
     deps = [":protobuf_python"],
 )
 
@@ -656,6 +658,7 @@ py_proto_library(
     include = "python",
     default_runtime = ":protobuf_python",
     protoc = ":protoc",
+    srcs_version = "PY2AND3",
     deps = [":python_common_test_protos"],
 )