Stop using Label.relative(), as it is deprecated

According to the Bazel documentation, this method is deprecated:

https://bazel.build/rules/lib/builtins/Label#relative

In this specific case there is also no need to use it, as we can simply
use .repo_name to obtain the repository's canonical name.

Author: EdSchouten

python/private/toolchains_repo.bzl

@@ -31,10 +31,6 @@ load(
 load(":repo_utils.bzl", "REPO_DEBUG_ENV_VAR", "repo_utils")
 load(":text_util.bzl", "render")
 
-def get_repository_name(repository_workspace):
-    dummy_label = "//:_"
-    return str(repository_workspace.relative(dummy_label))[:-len(dummy_label)] or "@"
-
 def python_toolchain_build_file_content(
         prefix,
         python_version,
@@ -90,7 +86,7 @@ def _toolchains_repo_impl(rctx):
 # python_register_toolchains macro so you don't normally need to interact with
 # these targets.
 
-load("@{rules_python}//python/private:py_toolchain_suite.bzl", "py_toolchain_suite")
+load("@@{rules_python}//python/private:py_toolchain_suite.bzl", "py_toolchain_suite")
 
 """.format(
         rules_python = rctx.attr._rules_python_workspace.repo_name,
@@ -151,13 +147,13 @@ toolchain_aliases(
     rctx.file("defs.bzl", content = """\
 # Generated by python/private/toolchains_repo.bzl
 
-load("{rules_python}//python:pip.bzl", _compile_pip_requirements = "compile_pip_requirements")
-load("{rules_python}//python/private:deprecation.bzl", "with_deprecation")
-load("{rules_python}//python/private:text_util.bzl", "render")
-load("{rules_python}//python:py_binary.bzl", _py_binary = "py_binary")
-load("{rules_python}//python:py_test.bzl", _py_test = "py_test")
+load("@@{rules_python}//python:pip.bzl", _compile_pip_requirements = "compile_pip_requirements")
+load("@@{rules_python}//python/private:deprecation.bzl", "with_deprecation")
+load("@@{rules_python}//python/private:text_util.bzl", "render")
+load("@@{rules_python}//python:py_binary.bzl", _py_binary = "py_binary")
+load("@@{rules_python}//python:py_test.bzl", _py_test = "py_test")
 load(
-    "{rules_python}//python/entry_points:py_console_script_binary.bzl",
+    "@@{rules_python}//python/entry_points:py_console_script_binary.bzl",
     _py_console_script_binary = "py_console_script_binary",
 )
 
@@ -185,7 +181,7 @@ def compile_pip_requirements(**kwargs):
 """.format(
         name = rctx.attr.name,
         python_version = rctx.attr.python_version,
-        rules_python = get_repository_name(rctx.attr._rules_python_workspace),
+        rules_python = rctx.attr._rules_python_workspace.repo_name,
     ))
 
 toolchain_aliases = repository_rule(
@@ -343,7 +339,7 @@ def compile_pip_requirements(**kwargs):
             repository_name = repository_name,
             name = rctx.attr.name,
             python_version = python_version,
-            rules_python = get_repository_name(rctx.attr._rules_python_workspace),
+            rules_python = rctx.attr._rules_python_workspace.repo_name,
         ))
         rctx.file("{}/BUILD.bazel".format(python_version), "")