DEP8 fix: pygopherd and py3 support

Hello,

squid is currently failing DEP8 tests in both Ubuntu and Debian. The reason is that pygopherd was removed from Debian unstable and Ubuntu 20.04. Ubuntu has no replacement for a gopher server, so I just dropped that test, and I'm doing the same for Debian.

Also, while going over this, I changed the test to be run with python3, given python2's deprecation.

I ran this branch under lxd in debian/sid and the test passed:

$ time autopkgtest -o debian-dep8 -U -s -B ./squid/ -- lxd images:debian/sid
...
Enabling module ssl.
See /usr/share/doc/apache2/README.Debian.gz on how to configure SSL and create self-signed certificates.
To activate the new configuration, you need to run:
  systemctl restart apache2
Enabling site default-ssl.
To activate the new configuration, you need to run:
  systemctl reload apache2
test_daemons (__main__.BasicTest)
Test daemon ... ok
test_ftp_proxy (__main__.BasicTest)
Test ftp ... ok
test_http_proxy (__main__.BasicTest)
Test http ... ok
test_https_proxy (__main__.BasicTest)
Test https ... ok
test_squidclient (__main__.BasicTest)
Test squidclient ... ok
test_zz_apparmor (__main__.BasicTest)
Test apparmor ... ok

----------------------------------------------------------------------
Ran 6 tests in 238.047s

OK
autopkgtest [17:48:45]: test squid: -----------------------]
autopkgtest [17:48:45]: test squid:  - - - - - - - - - - results - - - - - - - - - -
squid                PASS
autopkgtest [17:48:46]: @@@@@@@@@@@@@@@@@@@@ summary
upstream-test-suite  PASS
squid                PASS

real	13m51,277s

Log attached log.gz