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.

--- a/execnet/__init__.py
+++ b/execnet/__init__.py
@@ -5,9 +5,9 @@
 """
 __version__ = '1.0.9'
 
-import execnet.apipkg
+import apipkg
 
-execnet.apipkg.initpkg(__name__, {
+apipkg.initpkg(__name__, {
     'PopenGateway':     '.deprecated:PopenGateway',
     'SocketGateway':    '.deprecated:SocketGateway',
     'SshGateway':       '.deprecated:SshGateway',
