py-pip: fix import tests (#41867)

var/spack/repos/builtin/packages/py-pip/package.py

@@ -15,6 +15,9 @@ class PyPip(Package, PythonExtension):
     url = "https://files.pythonhosted.org/packages/py3/p/pip/pip-20.2-py3-none-any.whl"
     list_url = "https://pypi.org/simple/pip/"
 
+    # Requires railroad
+    skip_modules = ["pip._vendor"]
+
     tags = ["build-tools"]
 
     maintainers("adamjstewart", "pradyunsg")