tox: Filter pytest to <2.8.0 for requires.io.

We'll have to wait for hypothesis-pytest and pytest-capturelog/catchlog to be
fixed first.

tox.ini

@@ -16,7 +16,7 @@ passenv = PYTHON DISPLAY XAUTHORITY HOME USERNAME USER CI
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.3
+    pytest==2.7.3  # rq.filter: <2.8.0
     pytest-capturelog==0.7
     pytest-qt==1.7.0
     pytest-mock==0.7.0
@@ -109,7 +109,7 @@ passenv =
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.3
+    pytest==2.7.3  # rq.filter: <2.8.0
     pyflakes==0.9.2
     pytest-flakes==1.0.1
 commands =
@@ -122,7 +122,7 @@ passenv =
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.3
+    pytest==2.7.3  # rq.filter: <2.8.0
     pep8==1.6.2
     pytest-pep8==1.0.6
 commands =
@@ -135,7 +135,7 @@ passenv =
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.3
+    pytest==2.7.3  # rq.filter: <2.8.0
     mccabe==0.3.1
     pytest-mccabe==0.1
 commands =