Provide current_py_toolchain (#731)

This commit introduces the current_py_toolchain rule, exposing the
PYTHON2 and PYTHON3 "make" variables in bazel rules, analagous to
@bazel_tools//tools/cpp:current_cc_toolchain.

See
https://docs.bazel.build/versions/main/be/make-variables.html#custom_variables

Co-authored-by: Thulio Ferraz Assis <3149049+f0rmiga@users.noreply.github.com>

python/BUILD

diff --git a/python/BUILD b/python/BUILD
index 05cc9d1..ce19653 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -24,6 +24,8 @@
 that @rules_python//python is only concerned with the core rules.
 """
 
+load(":defs.bzl", "current_py_toolchain")
+
 package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
@@ -138,3 +140,7 @@
     "packaging.bzl",
     "pip.bzl",
 ])
+
+current_py_toolchain(
+    name = "current_py_toolchain",
+)