Remove .tox directory from setuptools own sdist (#4602)

Author: abravalheri

MANIFEST.in

@@ -19,3 +19,4 @@ include tox.ini
 include setuptools/tests/config/setupcfg_examples.txt
 include setuptools/config/*.schema.json
 global-exclude *.py[cod] __pycache__
+prune .tox

setuptools/tests/test_sdist.py

@@ -907,3 +907,12 @@ def run_sdist(monkeypatch, project):
     archive = next((project / "dist").glob("*.tar.gz"))
     with tarfile.open(str(archive)) as tar:
         return set(tar.getnames())
+
+
+def test_sanity_check_setuptools_own_sdist(setuptools_sdist):
+    with tarfile.open(setuptools_sdist) as tar:
+        files = tar.getnames()
+
+    # setuptools sdist should not include the .tox folder
+    tox_files = [name for name in files if ".tox" in name]
+    assert len(tox_files) == 0