stack/slapos: Change the version number of pytest-runner from 5.2.0 to 5.2

Use the exact version number displayed on the pypi index, because our shacache system is based on the URL which contains a package version number.

stack/slapos.cfg

@@ -228,7 +228,7 @@ feedparser = 5.2.1
 functools32 = 3.2.3.post2
 attrs = 18.2.0
 pyrsistent = 0.16.1
-pytest-runner = 5.2.0:whl
+pytest-runner = 5.2:whl
 ipaddress = 1.0.23
 jsonschema = 3.0.2:whl
 lockfile = 0.12.2:whl