Psycopg bug tracker moved to GitHub

Hello,

I've moved the psycopg bug tracker from Lighthouse to GitHub to get a
better integration with the source repository and to provide bug
reporter a tool they are more used to.

New bugs can be submitted at
<https://github.com/psycopg/psycopg2/issues>. The ticket from the
previous tracker have been imported into the GitHub tracker
maintaining the original id (except the older ones which had to be
renamed because their number conflicted with "pull requests" created
in GitHub). The previous tickets are still available on the Lighthouse
website but they are in read-only mode now.

I've fixed the documentation and the website to point at the tickets
in the new tracker, but if you find outdated links please let me know
and I'll try to fix them.

Thank you very much to the Lighthouse team for providing their service
and support during these years.

-- Daniele