Drop Python 2.7 support / Require Python 3.10

[peace-maker]

Only test on Python 3 and bump minimal required Python version to 3.10. Stop testing on Python 2.7.

The support plan for Python versions looks like it's worth to support 3.10 if we're dropping support for Python versions which reached end-of-life anyways. Is 3.10 ok or should we support lower Python 3 versions? If we keep the required-python pyproject.toml field in sync, older Python versions should just install an older pwntools version automatically.

And so it begins @Arusekk

Refs #2495

[Arusekk]

I guess we can use 3.4 for the machine-readable requirements, since we're just dropping these select workarounds. But we can state 3.10 in readme, that's okay. (3.10 supported vs 3.4 required)

[peace-maker]

We'd need to test on 3.4 though since we might start using newer Python3 features now like f-strings or type-hints to make sure we stay compatible. Otherwise we'll drift away from 3.4 unnoticed. We can postpone that decision once we are at the point of wanting to use newer features that aren't backported of course. Do you have other reasons to stay at 3.4 or some other earlier Python3 version?

vermin can help to monitor our minimum required version. Right now we're already requiring 3.5 at minimum on some code paths:

vermin --target=3.4 --exclude-regex 'pwnlib/data' -vvv pwnlib | grep '3\
.5'
2.7, 3.5     /home/jannik/tools/pwntools/pwnlib/fmtstr.py
  L946: bytes `%` formatting or `str` synonym requires 2.6, 3.5
  L980: bytes `%` formatting or `str` synonym requires 2.6, 3.5
!2, 3.5      /home/jannik/tools/pwntools/pwnlib/gdb.py
  L346: bytes `%` formatting or `str` synonym requires 2.6, 3.5
  L348: bytes `%` formatting or `str` synonym requires 2.6, 3.5
  L1130: bytes `%` formatting or `str` synonym requires 2.6, 3.5
2.6, 3.5     /home/jannik/tools/pwntools/pwnlib/protocols/adb/__init__.py
  L48: bytes `%` formatting or `str` synonym requires 2.6, 3.5
  L534: bytes `%` formatting or `str` synonym requires 2.6, 3.5
2.7, 3.5     /home/jannik/tools/pwntools/pwnlib/tubes/ssh.py
  L89: bytes `%` formatting or `str` synonym requires 2.6, 3.5
  L1482: bytes `%` formatting or `str` synonym requires 2.6, 3.5
  L1594: bytes `%` formatting or `str` synonym requires 2.6, 3.5
!2, 3.5      /home/jannik/tools/pwntools/pwnlib/util/misc.py
  L350: 'subprocess.run' member requires !2, 3.5
2.6, 3.5     /home/jannik/tools/pwntools/pwnlib/util/sh_string.py
  L488: bytes `%` formatting or `str` synonym requires 2.6, 3.5
Minimum required versions: 3.5

[Arusekk]

Sure, we can do 3.8 as well (introduced some major parts of type hints)

[peace-maker]

After some discussion on Discord the suggestion was setting the required version to install as low as possible and only update the minimum version once we really want to use some language feature. I'd be fine with starting with 3.4 and see if that helps cleanup the code at all. But state 3.4 as the minimum required version in the docs too so contributers know what to expect.

We should setup vermin in CI to monitor when we change our target version. I can fix those few occurences of 3.5 features for now.

[peace-maker]

Hm, it wasn't possible to install pwntools on Python < 3.6 since version 4.11 for over a year now due to requiring unix_ar and nobody complained. Download stats on pypi show some usage on Python 3.4 and 3.5 though so I'd assume an older version is pinned there. Maybe 3.6 is a compromise to start on then? We'll have to see if this really helps with the maintenance work since it's EOL too for 3 years -.-