bazel: Mark Python default version different (#530)

With this way, we are more compatible to also different rules_python
versions.

Author: castler

MODULE.bazel

@@ -13,13 +13,8 @@ bazel_dep(name = "rules_python_gazelle_plugin", version = "1.5.1")
 
 python = use_extension("@rules_python//python/extensions:python.bzl", "python")
 
-python.defaults(
-    # Use python.defaults if you have defined multiple toolchain versions.
-    python_version = "3.9",
-    python_version_env = "BAZEL_PYTHON_VERSION",
-)
-
 python.toolchain(
+    is_default = True,
     configure_coverage_tool = True,
     python_version = "3.9",
 )