fix: add missing python version flag attr to pex (#399)

Adds the missing `_interpreter_version_flag` attr to the pex binary rule
that allows it to resolve the correct Python toolchain in older versions
of bazel.

---

### 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): no
- Suggested release notes appear below:

Ensure `py_pex_binary` can locate the Python version on the toolchain
correctly on older bazel versions.

### Test plan

- Manual testing; please provide instructions so we can reproduce:

Run test suite with bazel 6.5.0

py/private/py_pex_binary.bzl

@@ -19,12 +19,12 @@ exclude_paths = [
     "aspect_rules_py~/py/tools/",
     # these will match in bzlmod setup with --incompatible_use_plus_in_repo_names flag flipped.
     "rules_python++python+",
-    "aspect_rules_py+/py/tools/"
+    "aspect_rules_py+/py/tools/",
 ]
 
 # determines if the given file is a `distinfo`, `dep` or a `source`
 # this required to allow PEX to put files into different places.
-# 
+#
 # --dep:        into `<PEX_UNPACK_ROOT>/.deps/<name_of_the_package>`
 # --distinfo:   is only used for determining package metadata
 # --source:     into `<PEX_UNPACK_ROOT>/<relative_path_to_workspace_root>/<file_name>`
@@ -37,23 +37,24 @@ def _map_srcs(f, workspace):
             return []
 
     site_packages_i = f.path.find("site-packages")
-    
+
     # if path contains `site-packages` and there is only two path segments
     # after it, it will be treated as third party dep.
     # Here are some examples of path we expect and use and ones we ignore.
-    #  
+    #
     # Match: `external/rules_python~~pip~pypi_39_rtoml/site-packages/rtoml-0.11.0.dist-info/INSTALLER`
     # Reason: It has two `/` after first `site-packages` substring.
-    # 
+    #
     # No Match: `external/rules_python~~pip~pypi_39_rtoml/site-packages/rtoml-0.11.0/src/mod/parse.py`
     # Reason: It has three `/` after first `site-packages` substring.
     if site_packages_i != -1 and f.path.count("/", site_packages_i) == 2:
         if f.path.find("dist-info", site_packages_i) != -1:
             return ["--distinfo={}".format(f.dirname)]
         return ["--dep={}".format(f.dirname)]
 
-    # If the path does not have a `site-packages` in it, then put it into 
-    # the standard runfiles tree.
+        # If the path does not have a `site-packages` in it, then put it into
+        # the standard runfiles tree.
+
     elif site_packages_i == -1:
         return ["--source={}={}".format(f.path, dest_path)]
 
@@ -74,22 +75,22 @@ def _py_python_pex_impl(ctx):
     workspace_name = str(ctx.workspace_name)
 
     args.add_all(
-        ctx.attr.inject_env.items(), 
+        ctx.attr.inject_env.items(),
         map_each = lambda e: "--inject-env={}={}".format(e[0], e[1]),
-        # this is needed to allow passing a lambda to map_each 
+        # this is needed to allow passing a lambda to map_each
         allow_closure = True,
     )
 
     args.add_all(
-        binary[PyInfo].imports, 
-        format_each = "--sys-path=%s"
+        binary[PyInfo].imports,
+        format_each = "--sys-path=%s",
     )
 
     args.add_all(
         runfiles.files,
         map_each = lambda f: _map_srcs(f, workspace_name),
         uniquify = True,
-        # this is needed to allow passing a lambda (with workspace_name) to map_each 
+        # this is needed to allow passing a lambda (with workspace_name) to map_each
         allow_closure = True,
     )
     args.add(binary[DefaultInfo].files_to_run.executable, format = "--executable=%s")
@@ -99,10 +100,10 @@ def _py_python_pex_impl(ctx):
     py_version = py_toolchain.interpreter_version_info
     args.add_all(
         [
-            constraint.format(major = py_version.major, minor = py_version.minor, patch = py_version.micro) 
+            constraint.format(major = py_version.major, minor = py_version.minor, patch = py_version.micro)
             for constraint in ctx.attr.python_interpreter_constraints
-        ], 
-        format_each = "--python-version-constraint=%s"
+        ],
+        format_each = "--python-version-constraint=%s",
     )
     args.add(output, format = "--output-file=%s")
 
@@ -116,10 +117,9 @@ def _py_python_pex_impl(ctx):
     )
 
     return [
-        DefaultInfo(files = depset([output]), executable = output)
+        DefaultInfo(files = depset([output]), executable = output),
     ]
 
-
 _attrs = dict({
     "binary": attr.label(executable = True, cfg = "target", mandatory = True, doc = "A py_binary target"),
     "inject_env": attr.string_dict(
@@ -128,7 +128,7 @@ _attrs = dict({
     ),
     "python_shebang": attr.string(default = "#!/usr/bin/env python3"),
     "python_interpreter_constraints": attr.string_list(
-        default = ["CPython=={major}.{minor}.*"], 
+        default = ["CPython=={major}.{minor}.*"],
         doc = """\
 Python interpreter versions this PEX binary is compatible with. A list of semver strings. 
 The placeholder strings `{major}`, `{minor}`, `{patch}` can be used for gathering version 
@@ -143,17 +143,21 @@ py_pex_binary
     ]
 )
 ```
-"""),
-    "_pex": attr.label(executable = True, cfg = "exec", default = "//py/tools/pex")
+""",
+    ),
+    # 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_pex_binary = rule(
     doc = "Build a pex executable from a py_binary",
     implementation = _py_python_pex_impl,
     attrs = _attrs,
     toolchains = [
-        PY_TOOLCHAIN
+        PY_TOOLCHAIN,
     ],
     executable = True,
-)
+)