ci: Remove Python 3.8; add Python 3.12.

[Niloth-p]

Fixes: Failing CI for Py 3.8
Fixes: #829
Fixes: #841 can be closed as well.

Removes Python 3.8.
Adds Py 3.11 Zulip 7.0 to matrix strategy.
Adds Python 3.12.

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[Niloth-p]

Self-reviewed.

[Niloth-p]

Earliest major Zulip version that has the same entry in its matrix.

[Niloth-p]

Earliest minor Zulip version that supports the platform is 8.3.

But, 8.3 and 8.4 are unable to use pip inside the venv, due to version conflicts related to pkg_resources. Only versions 8.5 onwards seem to be working.

Traceback

Traceback (most recent call last):
  File "/srv/zulip-py3-venv/bin/pip", line 5, in <module>
    from pip._internal.cli.main import main
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/cli/main.py", line 10, in <module>
    from pip._internal.cli.autocompletion import autocomplete
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/cli/autocompletion.py", line 9, in <module>
    from pip._internal.cli.main_parser import create_main_parser
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/cli/main_parser.py", line 7, in <module>
    from pip._internal.cli import cmdoptions
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/cli/cmdoptions.py", line 25, in <module>
    from pip._internal.cli.progress_bars import BAR_TYPES
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/cli/progress_bars.py", line 12, in <module>
    from pip._internal.utils.logging import get_indentation
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/utils/logging.py", line 18, in <module>
    from pip._internal.utils.misc import ensure_dir
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_internal/utils/misc.py", line 21, in <module>
    from pip._vendor import pkg_resources
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3254, in <module>
    @_call_aside
     ^^^^^^^^^^^
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3238, in _call_aside
    f(*args, **kwargs)
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3280, in _initialize_master_working_set
    tuple(
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3281, in <genexpr>
    dist.activate(replace=False)
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2788, in activate
    declare_namespace(pkg)
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2285, in declare_namespace
    _handle_ns(packageName, path_item)
  File "/srv/zulip-venv-cache/e2c2c053635e882ff393067adf908813bcad92ef/zulip-py3-venv/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2202, in _handle_ns
    loader = importer.find_module(packageName)
             ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'FileFinder' object has no attribute 'find_module'

[Niloth-p]

Though, the Zulip workflow mentions 3.12.2, the actual version used now seems to be 3.12.3.

[Niloth-p]

Fix for #829, included in this commit as it is necessary for the Py 3.12 tests (but not for the 3.11 one).

[timabbott]

Hmm, isn't the point of #829 that we can just drop this dependency and use the built-in module in 3.10? Or do we need to wait until we drop 3.9 support to drop it?

[Niloth-p]

Yes, we can't drop it before dropping 3.9 support.
We could conditionally use the built-in module for >=3.10 without waiting until the dependency is dropped though.

[timabbott]

Seems OK, merged, thanks @Niloth-p!