dev-requirements.txt: Remove obsolete comments

This also fixes an error with pylint in setup.py:
setup.py:226:19: R1718: Consider using a set comprehension (consider-using-set-comprehension)

dev-requirements.txt

-# Pin coverage to 4.2 for now, we're experiencing
-# random crashes with 4.4.2
 coverage == 4.4.0
 pep8
 pytest >= 3.1.0
@@ -8,5 +6,4 @@ pytest-datafiles
 pytest-env
 pytest-pep8
 pytest-pylint
-# Provide option to run tests in parallel, less reliable
 pytest-xdist

setup.py

@@ -221,10 +221,8 @@ def get_cmdclass():
 #####################################################
 #               Gather requirements                 #
 #####################################################
-setup_requires = set(['pytest-runner'])
 with open('dev-requirements.txt') as dev_reqs:
-    dev_requires = set([line for line in dev_reqs.read().split('\n')
-                        if not line.strip().startswith('#')])
+    dev_requires = dev_reqs.read().splitlines()
 
 
 #####################################################
@@ -269,6 +267,6 @@ setup(name='BuildStream',
           'grpcio >= 1.10',
       ],
       entry_points=bst_install_entry_points,
-      setup_requires=list(setup_requires),
-      tests_require=list(dev_requires - setup_requires),
+      setup_requires=['pytest-runner'],
+      tests_require=dev_requires,
       zip_safe=False)