TST make it clear which requirements are Python 2-only

* rename requirements.txt to requirements-py2.txt, to make it clear they are Python 2-only
* make requirements-py3.txt consistent with requirements-py2.txt

requirements-py2.txt

@@ -2,9 +2,9 @@ Twisted>=13.1.0
 lxml
 pyOpenSSL
 cssselect>=0.9
-w3lib>=1.17.0
 queuelib
+w3lib>=1.17.0
 six>=1.5.2
 PyDispatcher>=2.0.5
-service_identity
 parsel>=1.4
+service_identity

requirements-py3.txt

@@ -4,4 +4,7 @@ pyOpenSSL>=0.13.1
 cssselect>=0.9
 queuelib>=1.1.1
 w3lib>=1.17.0
+six>=1.5.2
+PyDispatcher>=2.0.5
+parsel>=1.4
 service_identity

tox.ini

@@ -9,13 +9,13 @@ envlist = py27
 [testenv]
 deps =
     -ctests/constraints.txt
-    -rrequirements.txt
+    -rrequirements-py2.txt
     # Extras
     botocore
     google-cloud-storage
     Pillow != 3.0.0
     leveldb
-    -rtests/requirements.txt
+    -rtests/requirements-py2.txt
 passenv =
     S3_TEST_FILE_URI
     AWS_ACCESS_KEY_ID
@@ -35,7 +35,7 @@ deps =
     Pillow==2.3.0
     cssselect==0.9.1
     zope.interface==4.0.5
-    -rtests/requirements.txt
+    -rtests/requirements-py2.txt
 
 [testenv:jessie]
 # https://packages.debian.org/en/jessie/python/
@@ -50,7 +50,7 @@ deps =
     Pillow==2.6.1
     cssselect==0.9.1
     zope.interface==4.1.1
-    -rtests/requirements.txt
+    -rtests/requirements-py2.txt
 
 [testenv:trunk]
 basepython = python2.7