Make reading of pu package lists optional

This allows the consideration of packages from proposed-updates to be
{dis,en}abled depending on whether the configuration file specifies
the path to the packages / sources files.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -279,14 +279,18 @@ class Britney:
         # lib/dpkg.c and rebuilding
         self.sources = {'testing': self.read_sources(self.options.testing),
                         'unstable': self.read_sources(self.options.unstable),
-                        'tpu': self.read_sources(self.options.tpu),
-                        'pu': self.read_sources(self.options.pu),}
+                        'tpu': self.read_sources(self.options.tpu),}
+        if 'pu' in self.options:
+            self.sources['pu'] = self.read_sources(self.options.pu)
+        else:
+            self.sources['pu'] = {}
         self.binaries = {'testing': {}, 'unstable': {}, 'tpu': {}, 'pu': {}}
         for arch in self.options.architectures:
             self.binaries['testing'][arch] = self.read_binaries(self.options.testing, "testing", arch)
             self.binaries['unstable'][arch] = self.read_binaries(self.options.unstable, "unstable", arch)
             self.binaries['tpu'][arch] = self.read_binaries(self.options.tpu, "tpu", arch)
-            self.binaries['pu'][arch] = self.read_binaries(self.options.pu, "pu", arch)
+            if 'pu' in self.options:
+                self.binaries['pu'][arch] = self.read_binaries(self.options.pu, "pu", arch)
             # build the testing system
             self.build_systems(arch)