This version of scons only support python2

See merge request !1140

This partially reverts ee733f9d because
of issues with python3

Commit ee733f9d (version up: zc.buildout 2.7.1+slapos017,
slapos.libnetworkcache 0.23, 2022-03-28) introduced a new way of
computing cache key used for shacache and downlaod URLs that were only
in cache but no longer available from their original location no longer
could be retrieved. This update the original download URLs to URLs that
are valid today.

See merge request nexedi/slapos!1147

I could not find tarball for 2.1 so I updated

also remove useless environment option which has same value as the macro

(+ small cosmetic fixes)

All instances in the wild have now moved their DB
from srv/res6stnet/registry to srv/re6stnet.

See commit a1ed9c30.

Fallback to alternate-url only happens on HTTPError.

SSL certificate of people.redhat.com has bad chain, which results with failed
downloads.

mysql: With universal_newlines, python3 subprocess assumes that communicate() parameter is str() (not bytes()).

But mysql_script_file was opened as a binary files and thus a bytes() was returned:
    File "etc/run/mariadb_update", line 51, in <module>
      sys.exit(slapos.recipe.generic_mysql.mysql.updateMysql(mysql_upgrade_binary='/srv/slapgrid/slappart3/bin/mysql_upgrade', mysql_binary='/srv/slapgrid/slappart3/bin/mysql', mysql_script_file='/srv/slapgrid/slappart3/etc/mariadb_initial_setup.sql'))
    File "parts/slapos.cookbook-repository/slapos/recipe/generic_mysql/mysql.py", line 31, in updateMysql
      result = mysql.communicate(mysql_script)[0]
    File "parts/python3/lib/python3.7/subprocess.py", line 964, in communicate
      stdout, stderr = self._communicate(input, endtime, timeout)
    File "parts/python3/lib/python3.7/subprocess.py", line 1692, in _communicate
      self._save_input(input)
    File "parts/python3/lib/python3.7/subprocess.py", line 1774, in _save_input
      self._input = self._input.encode(self.stdin.encoding,
  AttributeError: 'bytes' object has no attribute 'encode'