Prepare use (separately) packaged version of apipkg instead of embedded one.
execnet/apipkg.{py,pyc} are deleted via debian/rules to avoid patch not
applying on upstream upgrades.

Index: execnet/execnet/__init__.py
===================================================================
--- execnet.orig/execnet/__init__.py	2011-02-26 21:57:16.000000000 +0100
+++ execnet/execnet/__init__.py	2011-02-26 21:57:59.000000000 +0100
@@ -5,9 +5,9 @@
 """
 __version__ = "1.0.8"
 
-import execnet.apipkg
+import apipkg
 
-execnet.apipkg.initpkg(__name__, {
+apipkg.initpkg(__name__, {
     'PopenGateway':     '.deprecated:PopenGateway',
     'SocketGateway':    '.deprecated:SocketGateway',
     'SshGateway':       '.deprecated:SshGateway',
