fix weakref issue seen on one buildbot test
fix weakref issue seen on one buildbot test

Cito/sqlalchemy,wujuguang/sqlalchemy,halfcrazy/sqlalchemy,pdufour/sqlalchemy,dstufft/sqlalchemy,halfcrazy/sqlalchemy,monetate/sqlalchemy,graingert/sqlalchemy,davidjb/sqlalchemy,monetate/sqlalchemy,pdufour/sqlalchemy,brianv0/sqlalchemy,graingert/sqlalchemy,itkovian/sqlalchemy,robin900/sqlalchemy,bdupharm/sqlalchemy,Akrog/sqlalchemy,wfxiang08/sqlalchemy,olemis/sqlalchemy,j5int/sqlalchemy,WinterNis/sqlalchemy,sqlalchemy/sqlalchemy,elelianghh/sqlalchemy,inspirehep/sqlalchemy,bdupharm/sqlalchemy,elelianghh/sqlalchemy,davidfraser/sqlalchemy,davidfraser/sqlalchemy,wujuguang/sqlalchemy,hsum/sqlalchemy,itkovian/sqlalchemy,robin900/sqlalchemy,alex/sqlalchemy,epa/sqlalchemy,alex/sqlalchemy,sandan/sqlalchemy,inspirehep/sqlalchemy,Akrog/sqlalchemy,bootandy/sqlalchemy,wfxiang08/sqlalchemy,WinterNis/sqlalchemy,j5int/sqlalchemy,epa/sqlalchemy,bootandy/sqlalchemy,EvaSDK/sqlalchemy,dstufft/sqlalchemy,brianv0/sqlalchemy,davidjb/sqlalchemy,ThiefMaster/sqlalchemy,Cito/sqlalchemy,EvaSDK/sqlalchemy,sandan/sqlalchemy,hsum/sqlalchemy,276361270/sqlalchemy,zzzeek/sqlalchemy,ThiefMaster/sqlalchemy,alex/sqlalchemy,Cito/sqlalchemy,276361270/sqlalchemy,olemis/sqlalchemy