exp(pypi): enable pipstar by default on bzlmod

DO NOT MERGE: this is still a test

Author: aignas

CHANGELOG.md

@@ -61,7 +61,8 @@ END_UNRELEASED_TEMPLATE
 
 {#v0-0-0-changed}
 ### Changed
-* Nothing changed.
+* (pypi) `pipstar` flag has been flipped to be enabled by default for bzlmod
+  ([#2949](https://github.com/bazel-contrib/rules_python/issues/2949)).
 
 {#v0-0-0-fixed}
 ### Fixed

python/private/internal_config_repo.bzl

@@ -20,8 +20,9 @@ settings for rules to later use.
 
 load(":repo_utils.bzl", "repo_utils")
 
+_BZLMOD_ENABLED = "@@" in str(Label("//:unused"))
 _ENABLE_PIPSTAR_ENVVAR_NAME = "RULES_PYTHON_ENABLE_PIPSTAR"
-_ENABLE_PIPSTAR_DEFAULT = "0"
+_ENABLE_PIPSTAR_DEFAULT = "1" if _BZLMOD_ENABLED else "0"
 _ENABLE_PYSTAR_ENVVAR_NAME = "RULES_PYTHON_ENABLE_PYSTAR"
 _ENABLE_PYSTAR_DEFAULT = "1"
 _ENABLE_DEPRECATION_WARNINGS_ENVVAR_NAME = "RULES_PYTHON_DEPRECATION_WARNINGS"