Remove unnecessary pytest-runner from setup_requires.

Modelled after similar patch to python3-mccabe.

Upstream-Status: Pending
---
diff -U3 -r flake8-3.5.0-orig/setup.py flake8-3.5.0/setup.py
--- flake8-3.5.0-orig/setup.py	2019-04-25 15:11:52.406978457 -0700
+++ flake8-3.5.0/setup.py	2019-04-25 15:12:38.102978457 -0700
@@ -142,6 +142,5 @@
         "Topic :: Software Development :: Libraries :: Python Modules",
         "Topic :: Software Development :: Quality Assurance",
     ],
-    tests_require=tests_require,
-    setup_requires=['pytest-runner'],
+    tests_require=tests_require
 )