[FYI] Post-release administrivia

[Stefano Lattarini]

I've pushed the v1.13 tag, done a fast-forward merge of master into
maint, bumped the version number in maint to 1.13.0a and in master to
1.13a, merged the new maint into the new master, and pushed out both
master and maint.

Regards,
  Stefano