monkeypatch.syspath_prepend: Skip fixup_namespace_packages if pkg_resources not imported

Calling pkg_resources.fixup_namespace_packages() is only needed for packages
that use pkg_resources.declare_namespace() and hence they already imported
pkg_resources. When pkg_resources is not imported, we don't need to use it.

This avoids an unneeded runtime dependency on setuptools.
The code is tested by test_syspath_prepend_with_namespace_packages,
behavior should remain unchanged, hence no new test was added.

When people drop pkg_resources from sys.modules, they are on their own.
If someone has a actual use case making this valid to support,
they can come in and provide a test, a reference and a fix.

changelog/8503.trivial.rst

@@ -0,0 +1,4 @@
+:meth:`pytest.MonkeyPatch.syspath_prepend` no longer fails when
+``setuptools`` is not installed.
+It now only calls :func:`pkg_resources.fixup_namespace_packages` if
+``pkg_resources`` was previously imported, because it is not needed otherwise.

src/_pytest/monkeypatch.py

@@ -312,13 +312,16 @@ class MonkeyPatch:
 
     def syspath_prepend(self, path) -> None:
         """Prepend ``path`` to ``sys.path`` list of import locations."""
-        from pkg_resources import fixup_namespace_packages
 
         if self._savesyspath is None:
             self._savesyspath = sys.path[:]
         sys.path.insert(0, str(path))
 
         # https://github.com/pypa/setuptools/blob/d8b901bc/docs/pkg_resources.txt#L162-L171
+        # this is only needed when pkg_resources was already loaded by the namespace package
+        if "pkg_resources" in sys.modules:
+            from pkg_resources import fixup_namespace_packages
+
             fixup_namespace_packages(str(path))
 
         # A call to syspathinsert() usually means that the caller wants to