Remove

• backports_abc
• backports_functools_lru_cache
• backports_shutil_get_terminal_size
• backports_ssl_match_hostname
• configparser
• enum34
• functools32
• future
• ipaddress
• pathlib2
• singledispatch
• subprocess32
• typing

Let's also remove sagenb, since it doesn't build with Python 3. That lets us remove

• flask
• flask_autoindex
• flask_babel
• flask_oldsessions
• flask_openid
• flask_silk
• sagenb
• twisted (it may be used by terminado when it is present, but terminado seems to function fine without it)
• werkzeug

Component: packages: standard

Keywords: sd109

Author: John Palmieri

Branch/Commit: 0578937

Reviewer: Dima Pasechnik, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/29754