docs: Add a few intersphinx links to the Python Packaging User Guide (#804)

* Add a few intersphinx links to the Python Packaging User Guide

* docs: fix forward reference

* Update _types.py

---------

Co-authored-by: Henry Schreiner <[email protected]>

Author: edgarrmondragon

docs/conf.py

@@ -40,6 +40,7 @@
 
 intersphinx_mapping = {
     'python': ('https://docs.python.org/3/', None),
+    'packaging': ('https://packaging.python.org/en/latest/', None),
 }
 
 # Add any paths that contain templates here, relative to this directory.

docs/index.rst

@@ -4,10 +4,11 @@
 build
 *****
 
-A simple, correct Python packaging build frontend.
+A simple, correct Python packaging :std:term:`build frontend <Build Frontend>`.
 
 build manages ``pyproject.toml``-based builds, invoking
-build-backend hooks as appropriate to build a distribution package.
+:std:term:`build-backend <Build Backend>` hooks as appropriate to build a
+:std:term:`distribution package <Distribution Package>`.
 It is a simple build tool and does not perform any dependency management.
 
 .. sphinx_argparse_cli::

src/build/_types.py

@@ -1,14 +1,19 @@
 from __future__ import annotations
 
 import os
+import sys
 import typing
 
 
 __all__ = ['ConfigSettings', 'Distribution', 'StrPath', 'SubprocessRunner']
 
 ConfigSettings = typing.Mapping[str, typing.Union[str, typing.Sequence[str]]]
 Distribution = typing.Literal['sdist', 'wheel', 'editable']
-StrPath = typing.Union[str, 'os.PathLike[str]']
+
+if typing.TYPE_CHECKING or sys.version_info > (3, 9):
+    StrPath = typing.Union[str, os.PathLike[str]]
+else:
+    StrPath = typing.Union[str, os.PathLike]
 
 if typing.TYPE_CHECKING:
     from pyproject_hooks import SubprocessRunner