Move reference to external workspace to OSS-specific file

BUILD

@@ -31,6 +31,7 @@ load(
     "grpc_cc_library",
     "grpc_generate_one_off_targets",
     "grpc_upb_proto_library",
+    "python_config_settings",
 )
 
 config_setting(
@@ -63,11 +64,6 @@ config_setting(
     values = {"cpu": "x64_windows_msvc"},
 )
 
-config_setting(
-    name = "python3",
-    flag_values = {"@bazel_tools//tools/python:python_version": "PY3"},
-)
-
 config_setting(
     name = "mac_x86_64",
     values = {"cpu": "darwin"},
@@ -78,6 +74,8 @@ config_setting(
     values = {"define": "GRPC_USE_CPP_STD_LIB=1"},
 )
 
+python_config_settings()
+
 # This should be updated along with build.yaml
 g_stands_for = "ganges"
 

bazel/grpc_build_system.bzl

@@ -342,3 +342,10 @@ def grpc_objc_library(
 def grpc_upb_proto_library(name, deps):
     upb_proto_library(name = name, deps = deps)
 
+
+def python_config_settings():
+    native.config_setting(
+        name = "python3",
+        flag_values = {"@bazel_tools//tools/python:python_version": "PY3"},
+    )
+