stack/slapos: add missing setup-eggs for pyrsistent

pyrsistent has a setup_require of pytest-runner. Buildout installs
setup_require from their latest version, regardless of [versions]. When a
new version of pytest-runner was released without python 2 support this
broke installation of software releases on python 2.

stack/slapos.cfg

@@ -92,6 +92,11 @@ recipe = zc.recipe.egg
 eggs = slapos.cookbook
 depends = ${slapos-cookbook-dependencies:eggs}
 
+[pyrsistent]
+recipe = zc.recipe.egg:custom
+egg = pyrsistent
+setup-eggs = pytest-runner
+
 [slapos-cookbook-dependencies]
 recipe = zc.recipe.egg
 eggs =
@@ -101,6 +106,7 @@ eggs =
   ${python-cachecontrol:egg}
   ${python-cliff:egg}
   ${python-cryptography:egg}
+  ${pyrsistent:egg}
   ${jsonschema:egg}
 
 # slapos.toolbox containing utilities
@@ -273,6 +279,8 @@ attrs = 18.2.0
 # jsonschema==3.0.2
 pyrsistent = 0.16.1
 
+pytest-runner = 5.2.0
+
 # Required by:
 # jsonschema==3.0.2
 setuptools-scm = 3.5.0