tox: Update pytest to 2.7.2 and pylib to 1.4.29.

pytest upstream changelog
=========================

- fix issue767: pytest.raises value attribute does not contain the exception
  instance on Python 2.6. Thanks Eric Siegerman for providing the test
  case and Bruno Oliveira for PR.

- Automatically create directory for junitxml and results log.
  Thanks Aron Curzon.

- fix issue713: JUnit XML reports for doctest failures.
  Thanks Punyashloka Biswal.

- fix issue735: assertion failures on debug versions of Python 3.4+
  Thanks Benjamin Peterson.

- fix issue114: skipif marker reports to internal skipping plugin;
  Thanks Floris Bruynooghe for reporting and Bruno Oliveira for the PR.

- fix issue748: unittest.SkipTest reports to internal pytest unittest plugin.
  Thanks Thomas De Schampheleire for reporting and Bruno Oliveira for the PR.

- fix issue718: failed to create representation of sets containing unsortable
  elements in python 2. Thanks Edison Gustavo Muenz

- fix issue756, fix issue752 (and similar issues): depend on py-1.4.29
  which has a refined algorithm for traceback generation.

py upstream changelog
=====================

- fix issue55: revert a change to the statement finding algorithm
  which is used by pytest for generating tracebacks.
  Thanks Daniel Hahler for initial analysis.

- fix pytest issue254 for when traceback rendering can't
  find valid source code.  Thanks Ionel Cristian Maries.

tox.ini

@@ -21,8 +21,8 @@ setenv = QT_QPA_PLATFORM_PLUGIN_PATH={envdir}/Lib/site-packages/PyQt5/plugins/pl
 passenv = PYTHON DISPLAY XAUTHORITY HOME
 deps =
     -r{toxinidir}/requirements.txt
-    py==1.4.28
-    pytest==2.7.1
+    py==1.4.29
+    pytest==2.7.2
     pytest-capturelog==0.7
     pytest-qt==1.4.0
     pytest-mock==0.6.0
@@ -90,8 +90,8 @@ commands = {envpython} -m pep257 scripts tests qutebrowser --ignore=D102,D103,D2
 setenv = LANG=en_US.UTF-8
 deps =
     -r{toxinidir}/requirements.txt
-    py==1.4.28
-    pytest==2.7.1
+    py==1.4.29
+    pytest==2.7.2
     pyflakes==0.9.2
     pytest-flakes==1.0.0
 commands =
@@ -101,8 +101,8 @@ commands =
 [testenv:pep8]
 deps =
     -r{toxinidir}/requirements.txt
-    py==1.4.28
-    pytest==2.7.1
+    py==1.4.29
+    pytest==2.7.2
     pep8==1.6.2
     pytest-pep8==1.0.6
 commands =
@@ -112,8 +112,8 @@ commands =
 [testenv:mccabe]
 deps =
     -r{toxinidir}/requirements.txt
-    py==1.4.28
-    pytest==2.7.1
+    py==1.4.29
+    pytest==2.7.2
     mccabe==0.3.1
     pytest-mccabe==0.1
 commands =