GnuPG can't talk to keyservers on Stretch
That's a know bug in current src:gnupg2: dirmngr gets IPv6 addresses from DNS, tries to connect but can't since Tor disables IPv6 communication by default. The easiest workaround is to use a .onion keyserver.
GnuPG: use the Tor OnionBalance hidden service pool (Closes: #12202).
dirmngr in Stretch is broken with use-tor unless one either enables IPv6
traffic in Tor, or configures a Tor onion service as the keyserver.