feat(core): switch to script bootstrap as a default

Work towards #2156
Work towards #2521
Relands #2858 and reverts #2968
Fixes #2983

Author: aignas

CHANGELOG.md

@@ -69,6 +69,14 @@ END_UNRELEASED_TEMPLATE
 * (bootstrap) For {obj}`--bootstrap_impl=system_python`, the sys.path order has
   changed from `[app paths, stdlib, runtime site-packages]` to `[stdlib, app
   paths, runtime site-packages]`.
+* If using the (deprecated) autodetecting/runtime_env toolchain, then the Python
+  version specified at build-time *must* match the Python version used at
+  runtime (the {obj}`--@rules_python//python/config_settings:python_version`
+  flag and the {attr}`python_version` attribute control the build-time version
+  for a target). If they don't match, dependencies won't be importable. (Such a
+  misconfiguration was unlikely to work to begin with; this is called out as an
+  FYI).
+* (rules) {obj}`--bootstrap_impl=script` is the default for non-Windows for bazel 8 and above.
 
 {#v0-0-0-fixed}
 ### Fixed

docs/api/rules_python/python/config_settings/index.md

@@ -245,8 +245,12 @@ Values:
 ::::{bzl:flag} bootstrap_impl
 Determine how programs implement their startup process.
 
+The default for this depends on the platform:
+* Windows: `system_python` (**always** used)
+* Other: `script`
+
 Values:
-* `system_python`: (default) Use a bootstrap that requires a system Python available
+* `system_python`: Use a bootstrap that requires a system Python available
   in order to start programs. This requires
   {obj}`PyRuntimeInfo.bootstrap_template` to be a Python program.
 * `script`: Use a bootstrap that uses an arbitrary executable script (usually a
@@ -269,6 +273,11 @@ instead.
 :::{versionadded} 0.33.0
 :::
 
+:::{versionchanged} VERSION_NEXT_FEATURE
+* The default for non-Windows changed from `system_python` to `script`.
+* On Windows, the value is forced to `system_python`.
+:::
+
 ::::
 
 ::::{bzl:flag} current_config

python/config_settings/BUILD.bazel

@@ -90,7 +90,7 @@ string_flag(
 
 rp_string_flag(
     name = "bootstrap_impl",
-    build_setting_default = BootstrapImplFlag.SYSTEM_PYTHON,
+    build_setting_default = BootstrapImplFlag.SCRIPT,
     override = select({
         # Windows doesn't yet support bootstrap=script, so force disable it
         ":_is_windows": BootstrapImplFlag.SYSTEM_PYTHON,