Remove dependency on pip-run.

The dependency chain of `pip-run` includes `pydantic-core`.
The problem is that newer versions `pydantic-core` are written in Rust
and don't have pre-compiled wheels available for all platforms (e.g.
freebsd). It might be the case it is not even currently possible to
compile it to some platforms due to the lack of availability of the
toolchain (seems to be the problem with cygwin).

See `jaraco/inflect#195` and `pydantic/pydantic-core#773`.

setup.cfg

@@ -66,7 +66,6 @@ testing =
 	jaraco.path>=3.2.0
 	build[virtualenv]
 	filelock>=3.4.0
-	pip_run>=8.8
 	ini2toml[lite]>=0.9
 	tomli-w>=1.0.0
 	pytest-timeout

setuptools/tests/test_editable_install.py

@@ -13,7 +13,6 @@
 
 import jaraco.envs
 import jaraco.path
-import pip_run.launch
 import pytest
 from path import Path as _Path
 
@@ -379,7 +378,7 @@ def test_editable_with_prefix(tmp_path, sample_project, editable_opts):
     site_packages.mkdir(parents=True)
 
     # install workaround
-    pip_run.launch.inject_sitecustomize(site_packages)
+    _addsitedir(site_packages)
 
     env = dict(os.environ, PYTHONPATH=str(site_packages))
     cmd = [
@@ -1038,6 +1037,16 @@ def install_project(name, venv, tmp_path, files, *opts):
     return project, out
 
 
+def _addsitedir(new_dir: Path):
+    """To use this function, it is necessary to insert new_dir in front of sys.path.
+    The Python process will try to import a ``sitecustomize`` module on startup.
+    If we manipulate sys.path/PYTHONPATH, we can force it to run our code,
+    which invokes ``addsitedir`` and ensure ``.pth`` files are loaded.
+    """
+    file = f"import site; site.addsitedir({os.fspath(new_dir)!r})\n"
+    (new_dir / "sitecustomize.py").write_text(file, encoding="utf-8")
+
+
 # ---- Assertion Helpers ----