Update test-requirements to use shouldbe from PyPI

DirectXMan12 · DirectXMan12 · commit a52e4ac28e2c · 2015-04-07T13:13:43.000-04:00
Previously, test-requirements.txt pulled shouldbe from GitHub.
Now, we pull from PyPI instead.
diff --git a/test-requirements.txt b/test-requirements.txt
@@ -1,7 +1,7 @@
 flake8
 nose
 nose_parameterized
-git+https://github.com/DirectXMan12/should_be.git
+shouldbe
 six
 Cython
 
