Move py3k requirements to requirements/py3k.txt.

requirements/py3k.txt

@@ -0,0 +1,4 @@
+six==1.9.0
+typing==3.5.0.1
+future==0.15.2
+modernize==0.5

tools/travis/setup-py3k

@@ -1,7 +1,4 @@
 #!/bin/bash
 set -e
 set -x
-pip install --no-deps future==0.15.2
-pip install --no-deps modernize==0.5
-pip install --no-deps six==1.10.0
-pip install --no-deps typing==3.5.0.1
+pip install --no-deps -r requirements/py3k.txt