chore(semantics): Remove 6.0 compat version flag (#701)

We don't need it on reasonable Bazels anymore, move for now to relying
on the rules_python version flag which we also use in the new UV
machinery.

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes
- Breaking change (forces users to change their own code or config): yes
- Suggested release notes appear below: yes

BREAKING: Support for Bazel 6.X dropped. Bazel 7 and the Python
`interpreter_version_info` machinery is now required.

### Test plan

- Covered by existing test cases

---------

Co-authored-by: aspect-marvin[bot] <[email protected]>

Author: arrdem

py/BUILD.bazel

@@ -1,12 +1,8 @@
 load("@aspect_bazel_lib//:bzl_library.bzl", "bzl_library")
-load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
 
-# For Bazel 6.x compatibility, since
-# PyRuntimeInfo shipped only with Bazel 7
-# Users can set, e.g. --@aspect_rules_py//py:interpreter_version=3.9.18
-string_flag(
+alias(
     name = "interpreter_version",
-    build_setting_default = "",
+    actual = "@rules_python//python/config_settings:python_version",
     visibility = ["//visibility:public"],
 )
 

py/private/py_binary.bzl

@@ -169,10 +169,6 @@ A collision can occur when multiple packages providing the same file are install
     "_runfiles_lib": attr.label(
         default = "@bazel_tools//tools/bash/runfiles",
     ),
-    # NB: this is read by _resolve_toolchain in py_semantics.
-    "_interpreter_version_flag": attr.label(
-        default = "//py:interpreter_version",
-    ),
     # Required for py_version attribute
     "_allowlist_function_transition": attr.label(
         default = "@bazel_tools//tools/allowlists/function_transition_allowlist",

py/private/py_pex_binary.bzl

@@ -156,10 +156,6 @@ The placeholder strings `{major}`, `{minor}`, `{patch}` can be used for gatherin
 information from the hermetic python toolchain.
 """,
     ),
-    # NB: this is read by _resolve_toolchain in py_semantics.
-    "_interpreter_version_flag": attr.label(
-        default = "//py:interpreter_version",
-    ),
     "_pex": attr.label(executable = True, cfg = "exec", default = "//py/tools/pex"),
 })
 

py/private/py_semantics.bzl

@@ -1,6 +1,5 @@
 """Functions to determine which Python toolchain to use"""
 
-load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load("//py/private/toolchain:types.bzl", "PY_TOOLCHAIN")
 
 _INTERPRETER_FLAGS = [
@@ -13,21 +12,6 @@ _INTERPRETER_FLAGS = [
     "-I",
 ]
 
-_MUST_SET_INTERPRETER_VERSION_FLAG = """\
-ERROR: Prior to Bazel 7.x, the python interpreter version must be explicitly provided.
-
-For example in `.bazelrc` with Bazel 6.4, add
-        
-    common --@aspect_rules_py//py:interpreter_version=3.9.18
-
-Bazel 6.3 and earlier didn't handle the `common` verb for custom flags.
-Repeat the flag to avoid discarding the analysis cache:
-
-    build --@aspect_rules_py//py:interpreter_version=3.9.18
-    fetch --@aspect_rules_py//py:interpreter_version=3.9.18
-    query --@aspect_rules_py//py:interpreter_version=3.9.18
-"""
-
 _MUST_SET_TOOLCHAIN_INTERPRETER_VERSION_INFO = """
 ERROR: In Bazel 7.x and later, the python toolchain py_runtime interpreter_version_info must be set \
 to a dict with keys "major", "minor", and "micro".
@@ -89,18 +73,8 @@ def _resolve_toolchain(ctx):
             if not hasattr(py3_toolchain.interpreter_version_info, attr):
                 fail(_MUST_SET_TOOLCHAIN_INTERPRETER_VERSION_INFO)
         interpreter_version_info = py3_toolchain.interpreter_version_info
-    elif ctx.attr._interpreter_version_flag[BuildSettingInfo].value:
-        # Back-compat for Bazel 6.
-        # Same code as rules_python:
-        # https://github.com/bazelbuild/rules_python/blob/76f1c76f60ccb536d3b3e2c9f023d8063f40bcd5/python/repositories.bzl#L109
-        major, minor, micro = ctx.attr._interpreter_version_flag[BuildSettingInfo].value.split(".")
-        interpreter_version_info = struct(
-            major = major,
-            minor = minor,
-            micro = micro,
-        )
     else:
-        fail(_MUST_SET_INTERPRETER_VERSION_FLAG)
+        fail(_MUST_SET_TOOLCHAIN_INTERPRETER_VERSION_INFO)
 
     return struct(
         toolchain = py3_toolchain,

py/private/py_unpacked_wheel.bzl

@@ -61,10 +61,6 @@ _attrs = {
         allow_single_file = [".whl"],
         mandatory = True,
     ),
-    # NB: this is read by _resolve_toolchain in py_semantics.
-    "_interpreter_version_flag": attr.label(
-        default = "//py:interpreter_version",
-    ),
 }
 
 py_unpacked_wheel = rule(

py/private/py_venv/py_venv.bzl

@@ -297,10 +297,6 @@ A collision can occur when multiple packages providing the same file are install
         doc = "Additional options to pass to the Python interpreter.",
         default = [],
     ),
-    # NB: this is read by _resolve_toolchain in py_semantics.
-    "_interpreter_version_flag": attr.label(
-        default = "//py:interpreter_version",
-    ),
     # Required for py_version attribute
     "_allowlist_function_transition": attr.label(
         default = "@bazel_tools//tools/allowlists/function_transition_allowlist",