🎨 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.

docs/changelog-fragments/501.packaging.rst

@@ -0,0 +1,6 @@
+The ``toml`` build time dependency has been replaced with
+``tomli`` -- by :user:`webknjaz`
+
+The ``tomli`` distribution is only pulled in under Python
+versions below 3.11. On 3.11 and higher, the standard
+library module :py:mod:`tomllib` is now used instead.

packaging/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 load_toml_from_string # 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 = load_toml_from_string(config_file)
  return config_toml['tool']['local']['cythonize']
 
 

packaging/pep517_backend/_compat.py

@@ -0,0 +1,11 @@
+"""Cross-interpreter compatibility shims."""
+
+try:
+ # Python 3.11+
+ from tomllib import loads as load_toml_from_string # noqa: WPS433
+except ImportError:
+ # before Python 3.11
+ from tomli import loads as load_toml_from_string # noqa: WPS433, WPS440
+
+
+__all__ = ('load_toml_from_string',) # noqa: WPS410

pyproject.toml

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