[bazel] add python 3.10 runtime pair

[aslonnie]

getting ready to run things on python 3.10

[gemini-code-assist]

Code Review

This pull request correctly adds the Bazel configuration for a Python 3.10 toolchain by mirroring the existing setup for Python 3.9. The changes are straightforward and look good. I have included one suggestion to enhance maintainability by refactoring the duplicated toolchain definitions into a macro, which could be addressed in a follow-up. As a reminder, for this new toolchain to be active, it will need to be registered in the WORKSPACE file.

[gemini-code-assist]

This block of code for Python 3.10 is a copy of the existing definitions for Python 3.9. While this is correct, it introduces code duplication. To improve maintainability and make it easier to add future Python versions, consider refactoring this into a macro.

For example, a macro could be defined in a .bzl file (e.g., bazel/python.bzl):

# In a .bzl file, e.g., bazel/python.bzl
def create_python_toolchain(version, interpreter):
    native.py_runtime(
        name = f"py{version}_runtime",
        interpreter = interpreter,
        python_version = "PY3",
        visibility = ["//visibility:private"],
    )

    native.py_runtime_pair(
        name = f"py{version}_runtime_pair",
        py2_runtime = None,
        py3_runtime = f":py{version}_runtime",
        visibility = ["//visibility:private"],
    )

    native.toolchain(
        name = f"py{version}_toolchain",
        exec_compatible_with = ["//:hermetic_python"],
        toolchain = f":py{version}_runtime_pair",
        toolchain_type = "@bazel_tools//tools/python:toolchain_type",
    )

Then in BUILD.bazel, you could replace the duplicated blocks for both Python 3.9 and 3.10 with calls to this macro. This could be a good follow-up improvement.