IPv6 may cause trouble

[ryantrinkle]

When running with ipv6 enabled, pypi2nix hangs for long periods while trying to connect. This may be a problem with my machine, my network, my ISP, or something else, but I'm mentioning it here at @garbas' request in case it is an issue with pypi2nix itself.

[garbas]

for last months i couldn't reproduce this. i'm going to close this ticket but i will reopen it if some networking issues occur.