Drop official support for Python 3.7, add Python 3.12

[mahmoud]

For 3.7, it's EOL this year, and 23.1.0 exists for those who want it. Also, ecosystem packages like Chameleon are dropping support, so it's either skip/remove tests, or move with the masses.

For 3.12, tests technically pass, but probably want to fix these before adding it to classifiers.

Mostly utcnow, then one in boltons and many more in Chameleon (which only affect the integration and is held back for the moment bc of this issue).

utcnow:

  /home/mahmoud/projects/clastic/.tox/py312/lib/python3.12/site-packages/clastic/meta.py:121: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    now = datetime.datetime.utcnow()

One from boltons:

  /home/mahmoud/projects/clastic/.tox/py312/lib/python3.12/site-packages/boltons/timeutils.py:441: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    EPOCH_NAIVE = datetime.utcfromtimestamp(0)

And a bunch of chameleon warnings:

  /home/mahmoud/projects/clastic/.tox/py312/lib/python3.12/site-packages/chameleon/template.py:17: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

  /home/mahmoud/projects/clastic/.tox/py312/lib/python3.12/site-packages/chameleon/compiler.py:1106: DeprecationWarning: ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead
    keys=[ast.Str(s=name) for name in names],

.tox/py312/lib/python3.12/site-packages/clastic/tests/test_chameleon_render.py: 13 warnings
  /home/mahmoud/projects/clastic/.tox/py312/lib/python3.12/site-packages/chameleon/codegen.py:230: DeprecationWarning: ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead
    super(TemplateCodeGenerator, self).visit(ast.Num(n=node.pos))

.tox/py312/lib/python3.12/site-packages/clastic/tests/test_chameleon_render.py: 12 warnings
  /usr/lib/python3.12/ast.py:587: DeprecationWarning: Attribute n is deprecated and will be removed in Python 3.14; use value instead
    return Constant(*args, **kwargs)