feat: enable rules_python Starlark implemention by default for Bazel 7+

This makes the base rules use the Starlark code in rules_python
("pystar") instead of what is built into Bazel 7.

The pystar implementation can be disabled by setting
`RULES_PYTHON_ENABLE_PYSTAR=0` in your environment or using
`--action_env`.

Earlier versions of Bazel continue to use the rules built into Bazel
regardless (the new implementation requires special APIs that only Bazel
7 has).

Work towards #1069

python/private/internal_config_repo.bzl

@@ -19,7 +19,7 @@ settings for rules to later use.
 """
 
 _ENABLE_PYSTAR_ENVVAR_NAME = "RULES_PYTHON_ENABLE_PYSTAR"
-_ENABLE_PYSTAR_DEFAULT = "0"
+_ENABLE_PYSTAR_DEFAULT = "1"
 
 _CONFIG_TEMPLATE = """\
 config = struct(