Stop using the d2to1-based pbr

pbr updated itself to remove the d2to1 dependency. One of the main
reasons for this is a direct upstream dependency on distribute, which
causes the distribute/setuptools merge bug to get triggered, which
breaks people's system if they install with pip < 1.4.

Remove d2to1 references, and also bump the required version requested of
pbr to a version late enough to no longer depend on d2to1.

Remove the version specifier from setup.py, because setup_requires has
no way to trigger a version requirement upgrade. The flat version should
be sufficient for the setup.py interface in perpetuity.

Remove the setup-hook from setup.cfg, as it is no longer needed.

Change-Id: I40d3507c77802d13fd894aa16714b2ed6aa7d59f

requirements.txt

@@ -1,5 +1,4 @@
-d2to1>=0.2.10,<0.3
+pbr>=0.5.21,<1.0
-pbr>=0.5,<0.6
 
 SQLAlchemy >= 0.6
 decorator

setup.cfg

@@ -17,10 +17,6 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 2.6
 
-[global]
-setup-hooks =
-    pbr.hooks.setup_hook
-
 [files]
 packages =
     migrate

setup.py

@@ -17,5 +17,5 @@
 import setuptools
 
 setuptools.setup(
-    setup_requires=['d2to1>=0.2.10,<0.3', 'pbr>=0.5.10,<0.6'],
+    setup_requires=['pbr'],
-    d2to1=True)
+    pbr=True)