Update to use the hermetic python toolchain.

Part of bazelbuild#20437.

MODULE.bazel

@@ -27,7 +27,7 @@ bazel_dep(name = "rules_java", version = "7.3.1")
 bazel_dep(name = "rules_graalvm", version = "0.10.3")
 bazel_dep(name = "rules_proto", version = "5.3.0-21.7")
 bazel_dep(name = "rules_jvm_external", version = "5.2")
-bazel_dep(name = "rules_python", version = "0.26.0")
+bazel_dep(name = "rules_python", version = "0.27.0")
 bazel_dep(name = "rules_testing", version = "0.0.4")
 bazel_dep(name = "googletest", version = "1.14.0", repo_name = "com_google_googletest")
 
@@ -243,7 +243,10 @@ use_repo(
 # =========================================
 
 python = use_extension("@rules_python//python/extensions:python.bzl", "python")
-python.toolchain(python_version = "3.8")
+python.toolchain(
+    is_default = True,
+    python_version = "3.8",
+)
 
 pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")
 pip.parse(
@@ -307,6 +310,7 @@ gvm.graalvm(
     version = "20.0.2",
 )
 use_repo(gvm, "graalvm_toolchains")
+
 register_toolchains("@graalvm_toolchains//:gvm")
 
 # =========================================