tox: Update pytest to 2.8.4.

- ``deprecated_call()`` now works when the deprecated function has been already
  called by another test in the same module.
- ``--pastebin`` option now works on Python 3.
- ``--pastebin`` now works correctly when captured output contains non-ascii
  characters.
- Fix another error when collecting with a nasty __getattr__().
- fix the summary printed when no tests did run.
- a number of documentation modernizations wrt good practices.

tox.ini

@@ -26,7 +26,7 @@ deps =
     parse==1.6.6
     parse-type==0.3.4
     py==1.4.31
-    pytest==2.8.3
+    pytest==2.8.4
     pytest-bdd==2.15.0
     pytest-catchlog==1.2.0
     pytest-cov==2.2.0
@@ -155,7 +155,7 @@ deps =
     execnet==1.4.1
     py==1.4.31
     pyflakes==1.0.0
-    pytest==2.8.3
+    pytest==2.8.4
     pytest-cache==1.0
     pytest-flakes==1.0.1
 commands =
@@ -170,7 +170,7 @@ deps =
     execnet==1.4.1
     pep8==1.6.2
     py==1.4.31
-    pytest==2.8.3
+    pytest==2.8.4
     pytest-cache==1.0
     pytest-pep8==1.0.6
 commands =
@@ -186,7 +186,7 @@ deps =
     execnet==1.4.1
     mccabe==0.3.1
     py==1.4.31
-    pytest==2.8.3
+    pytest==2.8.4
     pytest-cache==1.0
     pytest-mccabe==0.1
 commands =