Swap the order of configparser imports

The configparser backport package has un-understood issues parsing
our files. But the intent of the code is to use whichever is the
package for the version of python in use. So, instead of trying to
install the python3 version first, which can also result in loading
the backport package, first try the python2 package, so that if
configparser itself is loaded, it will be because we're running python3.

Change-Id: I7ef497127822d0110804b04a674f24c195a07278

pbr/tests/util.py

@@ -44,9 +44,9 @@ import shutil
 import stat
 
 try:
-    import configparser
-except ImportError:
     import ConfigParser as configparser
+except ImportError:
+    import configparser
 
 
 @contextlib.contextmanager

pbr/util.py

@@ -73,9 +73,9 @@ from setuptools.dist import Distribution
 from setuptools.extension import Extension
 
 try:
-    import configparser
-except ImportError:
     import ConfigParser as configparser
+except ImportError:
+    import configparser
 
 import pbr.hooks