requirements: use latest pytest-cov, not blocked by py32 any more

also: pytest-benchmark 3.x is released, just use latest

requirements.d/development.txt

@@ -1,6 +1,6 @@
 virtualenv<14.0
 tox
 pytest
-pytest-cov<2.0.0
-pytest-benchmark>=3.0.0
+pytest-cov
+pytest-benchmark
 Cython