Add missing pathlib requirement for py3.3 support; re-enable 3.3 testing

.travis.yml

@@ -4,6 +4,7 @@ cache:
   directories:
     - $HOME/.cache/pip
 python:
+  - 3.3
   - 3.4
   - 3.5
   - 3.6

CHANGES.md

@@ -1,7 +1,10 @@
+v3.0.7 (dev)
+------------
+
 v3.0.6 (2017.07.22)
 -------------------
-* Added 'list_extensions' method to xphyle.formats.Formats
-* Fixed subtle bug that would cause failure when calling xopen on stdout that has been monkeypatched (as is done by pytest)
+* Added 'list_extensions' method to xphyle.formats.Formats.
+* Fixed subtle bug that would cause failure when calling xopen on stdout that has been monkeypatched (as is done by pytest).
 
 v3.0.5 (2017.07.19)
 -------------------

setup.py

@@ -2,11 +2,16 @@
 import sys
 
 requirements = []
+version_info = sys.version_info
 
-if sys.version_info < (3, 3):
+if version_info < (3, 3):
     sys.stdout.write("At least Python 3.3 is required.\n")
     sys.exit(1)
-elif sys.version_info < (3, 6):
+
+if version_info < (3, 4):
+    requirements.append('pathlib')
+
+if version_info < (3, 6):
     # typing was added in 3.5, and we rely on critical features that were
     # introduced in 3.5.2+, so for versions older than 3.6 we rely on
     # a backport