Add minimal reproduction with rules_python 1.6.3 (works)

This setup demonstrates the working state with rules_python 1.6.3.
Building gRPC's cython target succeeds:

  bazel build @com_github_grpc_grpc//src/python/grpcio/grpc/_cython:cygrpc.pyx_cython_translation

Author: thomasdesr

.bazelrc

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+# Use WORKSPACE mode, not bzlmod
+common --enable_bzlmod=false

.bazelversion

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+7.4.1

.gitignore

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+# Bazel
+bazel-*
+MODULE.bazel
+MODULE.bazel.lock

BUILD.bazel

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+# Empty BUILD file for workspace root

WORKSPACE

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+workspace(name = "rules_python_3495_repro")
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+# rules_python 1.6.3 - works fine
+http_archive(
+    name = "rules_python",
+    sha256 = "2f5c284fbb4e86045c2632d3573fc006facbca5d1fa02976e89dc0cd5488b590",
+    strip_prefix = "rules_python-1.6.3",
+    url = "https://github.com/bazel-contrib/rules_python/releases/download/1.6.3/rules_python-1.6.3.tar.gz",
+)
+
+load("@rules_python//python:repositories.bzl", "py_repositories", "python_register_toolchains")
+
+py_repositories()
+
+python_register_toolchains(
+    name = "python3_9",
+    python_version = "3.9",
+)
+
+# gRPC - its cython build triggers the issue
+http_archive(
+    name = "com_github_grpc_grpc",
+    sha256 = "afbc5d78d6ba6d509cc6e264de0d49dcd7304db435cbf2d630385bacf49e066c",
+    strip_prefix = "grpc-1.68.2",
+    urls = ["https://github.com/grpc/grpc/archive/refs/tags/v1.68.2.tar.gz"],
+)
+
+load("@com_github_grpc_grpc//bazel:grpc_deps.bzl", "grpc_deps")
+
+grpc_deps()