BuildDependsPolicy: handle old python3-apt

Very old versions, such as that run on the Ubuntu archive machine, don't
have the arch parameter.

Handle that by setting APT::Architecture instead

britney.py

@@ -298,6 +298,7 @@ class Britney(object):
 
         # initialize the apt_pkg back-end
         apt_pkg.init()
+        apt_pkg.init_config()
 
         # parse the command line arguments
         self._policy_engine = PolicyEngine()

britney2/policies/policy.py

@@ -1074,7 +1074,14 @@ class BuildDependsPolicy(BasePolicy):
             arch_results[arch] = BuildDepResult.OK
             # for every dependency block (formed as conjunction of disjunction)
             for block_txt in deps.split(','):
+                try:
                     block = parse_src_depends(block_txt, False, arch)
+                except TypeError:
+                    # old python3-apt didn't have the third argument
+                    native_arch = apt_pkg.config["APT::Architecture"]
+                    apt_pkg.config["APT::Architecture"] = arch
+                    block = parse_src_depends(block_txt, False)
+                    apt_pkg.config["APT::Architecture"] = native_arch
                 # Unlike regular dependencies, some clauses of the Build-Depends(-Arch|-Indep) can be
                 # filtered out by (e.g.) architecture restrictions.  We need to cope with this while
                 # keeping block_txt and block aligned.