add development.txt and modify tox.ini

requirements.d/development.txt

@@ -0,0 +1,4 @@
+tox
+mock
+pytest
+Cython

tox.ini

@@ -19,9 +19,7 @@ envlist = py32, py33, py34
 # Change dir to avoid import problem for cython code. The directory does
 # not really matter, should be just different from the toplevel dir.
 changedir = {toxworkdir}
-deps =
-    pytest
-    mock
+deps = -rrequirements.d/development.txt
 commands = py.test --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *