[test] Test develop of PEP660 egg w/o packages

This test is skipped for Python2 because pip and setuptools versions
compatible with Python2 do not support PEP 660 (i.e. editable installs
without setup.py).

src/zc/buildout/tests/test_all.py

@@ -1155,6 +1155,47 @@ existing setup.cfg:
 
 """
 
+def develop_pyproject_only_without_package():
+    """
+
+Install a pyproject-only project that does not contain any package,
+but only a single module.
+
+    >>> mkdir('nopkg')
+    >>> write('nopkg', 'pyproject.toml', '''
+    ... ''')
+
+    >>> write('nopkg', 'xnopkg.py', '''
+    ... print('nopkg')
+    ... ''')
+
+    >>> write('buildout.cfg', '''
+    ... [buildout]
+    ... develop = nopkg
+    ... parts =
+    ... ''')
+
+    >>> print_(system((join('bin', 'buildout'), '-vv')), end='')
+    Develop: '/sample-buildout/nopkg'
+    Found no package import path in develop project /sample-buildout/nopkg for packages ('xnopkg',)
+    Falling back to project's root directory
+
+    >>> ls('develop-eggs')
+    -  xnopkg.egg-link
+    -  zc.recipe.egg.egg-link
+    >>> cat('develop-eggs/xnopkg.egg-link')
+    /sample-buildout/nopkg
+
+"""
+# BBB Python2: skip PEP 660 test
+# Whether the test succeeds actually depends on pip and setuptools versions
+# supporting PEP 660, but Python2 is the reason they would be stuck at old
+# versions, so it will always fail with Python2, and Python3 should use more
+# recent versions.
+if sys.version_info < (3,):
+  del develop_pyproject_only_without_package
+
+
 def uninstall_recipes_used_for_removal():
     r"""
 Uninstall recipes need to be called when a part is removed too: