revert(pypi): revert the default for pipstar (#3373)

A user mentioned that flipping the pipstar default caused
a regression in their setup in [this comment]. Until we have
a better understanding and a regression test for that, we should
revert the flip. Adjusted the CHANGELOG notes for this.

Related to #2949

[this comment]:
#2949 (comment)

(cherry picked from commit 3c3e0be)

Author: aignas

CHANGELOG.md

@@ -74,10 +74,10 @@ END_UNRELEASED_TEMPLATE
 * (toolchains) `py_runtime` and `PyRuntimeInfo` reject Python 2 settings.
   Setting `py_runtime.python_version = "PY2"` or non-None
   `PyRuntimeInfo.py2_runtime` is an error.
-* (pypi) `pipstar` flag has been flipped to be enabled by default, to turn it
-  off use `RULES_PYTHON_ENABLE_PIPSTAR=0` environment variable. If you do, please
+* (pypi) `pipstar` flag has been implemented for `WORKSPACE` and can be flipped to be enabled using `RULES_PYTHON_ENABLE_PIPSTAR=1` environment variable. If you do, please
   add a comment to
-  [#2949](https://github.com/bazel-contrib/rules_python/issues/2949).
+  [#2949](https://github.com/bazel-contrib/rules_python/issues/2949) if you run into any
+  problems.
   With this release we are deprecating {obj}`pip.parse.experimental_target_platforms` and
   {obj}`pip_repository.experimental_target_platforms`. For users using `WORKSPACE` and
   vendoring the `requirements.bzl` file, please re-vendor so that downstream is unaffected

python/private/internal_config_repo.bzl

@@ -22,7 +22,7 @@ load("//python/private:text_util.bzl", "render")
 load(":repo_utils.bzl", "repo_utils")
 
 _ENABLE_PIPSTAR_ENVVAR_NAME = "RULES_PYTHON_ENABLE_PIPSTAR"
-_ENABLE_PIPSTAR_DEFAULT = "1"
+_ENABLE_PIPSTAR_DEFAULT = "0"
 _ENABLE_DEPRECATION_WARNINGS_ENVVAR_NAME = "RULES_PYTHON_DEPRECATION_WARNINGS"
 _ENABLE_DEPRECATION_WARNINGS_DEFAULT = "0"