🎨 Migrate PEP 517 backend to tomllib+tomli

`tomllib` is a part of stdlib since Python 3.11 and `tomli` is used as
a fallback for the older Python versions. The latter is API-compatible
with the former.

bin/pep517_backend/_backend.py

@@ -8,7 +8,6 @@
 from functools import wraps
 from pathlib import Path
 
-import toml
 from expandvars import expandvars
 from setuptools.build_meta import (  # noqa: F401  # Re-exporting PEP 517 hooks
     build_sdist, build_wheel, get_requires_for_build_sdist,
@@ -22,12 +21,17 @@
 
 from Cython.Build.Cythonize import main as cythonize_cli_cmd
 
+from ._compat import tomllib  # noqa: WPS436
 from ._transformers import (  # noqa: WPS436
     convert_to_kwargs_only, get_cli_kwargs_from_config,
     get_enabled_cli_flags_from_config,
 )
 
 
+PROJECT_ROOT_DIR = Path(__file__).parents[2].resolve()
+PYPROJECT_TOML_PATH = PROJECT_ROOT_DIR / 'pyproject.toml'
+
+
 def get_config():
     """Grab optional build dependencies from pyproject.toml config.
 
@@ -80,8 +84,8 @@ def get_config():
         # This section can contain cythonize options
         # NAME = "VALUE"
     """
-    config_file = (Path.cwd().resolve() / 'pyproject.toml').read_text()
-    config_toml = toml.loads(config_file)
+    config_file = PYPROJECT_TOML_PATH.read_text(encoding='utf-8')
+    config_toml = tomllib.loads(config_file)
     return config_toml['tool']['local']['cythonize']
 
 

bin/pep517_backend/_compat.py

@@ -0,0 +1,9 @@
+"""Cross-interpreter compatibility shims."""
+
+try:
+    import tomllib  # Python 3.11+  # noqa: WPS433
+except ImportError:
+    import tomli as tomllib  # before Python 3.11  # noqa: WPS433, WPS440
+
+
+__all__ = ('tomllib',)  # noqa: WPS410

pyproject.toml

@@ -3,7 +3,7 @@ requires = [
   # Essentials
   "Cython",  # needed by in-tree build backend `bin/pep517_backend.py`
   "setuptools>=45",  # needed by in-tree build backend `bin/pep517_backend.py`
-  "toml",  # needed by in-tree build backend `bin/pep517_backend.py`
+  "tomli; python_version < '3.11'",  # needed by in-tree build backend `bin/pep517_backend.py`
   "expandvars",  # needed by in-tree build backend for env vars interpolation
 
   # Plugins