dev_requirements.txt: bump minimum pytest versions to a working set

Not all combinations of pytest-related packages seem to work.
Trial-and-error lead to these combinations of minimum versions.

Add a minimum bound to pytest-benchmark and pytest-localserver to help pip
in figuring out a suitable combination with pytest, and also to allow
testing minimum combinations in the future.

Drop pytest-runner, which should not be needed unless when running tests
directly from setup.py.

dev_requirements.txt

@@ -1,8 +1,7 b''
-pytest >= 3.3.0, < 4.7
-pytest-runner < 5.2
-pytest-sugar >= 0.7.0, < 0.10
-pytest-benchmark < 3.3
-pytest-localserver < 0.6
+pytest >= 4.6.6, < 4.7
+pytest-sugar >= 0.9.2, < 0.10
+pytest-benchmark >= 3.2.2, < 3.3
+pytest-localserver >= 0.5.0, < 0.6
 mock < 3.1
 Sphinx < 1.9
 WebTest < 2.1