Remove old backports & python 2 shims

[Tom-Willemsen]

Some candidate modules:

• 2to3
• backports-abc
• backports.functools-lru-cache
• backports.shutil-get-terminal-size
• contextlib2
• ipython-genutils
• mock (use unittest.mock instead)
• future (review usages - are any of them recent enough to still be needed?)

Time in recording of planning meeting: 00h30m 2024/11/28

[davidkeymer]

This ticket will be completed as-and-when and a Flash Review created.