Use `from __future__ import annotations` in every module, enforced by ruff

[oberstet]

Why

Twice now, a type annotation that is fine for type checkers broke import autobahn... at runtime
on CPython 3.11–3.13, and both times it shipped:

• [BUG] 26.6.1: import autobahn.wamp.cryptosign fails with TypeError on CPython 3.11–3.13 (regression from #1843) #1878: a string forward reference combined with | None ("ISecurityModule" | None) raised
  TypeError at class-definition time in autobahn.wamp.cryptosign.
• [BUG] autobahn.twisted import fails on CPython 3.11–3.13 with zope.interface < 6.2 (gap in #1878) #1955: ITransport | IProcessTransport raised TypeError in autobahn.twisted.util / .wamp,
  because zope.interface's InterfaceClass supports | only from 6.2 on (fixed by Update zope.interface dependency and add future annotations import #1959).

Both happen because, before Python 3.14, annotations on module-level functions and classes are
evaluated eagerly when the def / class statement runs. Any expression in an annotation that
does not work at runtime on that Python, with those dependency versions, becomes an import error.
from __future__ import annotations (PEP 563) makes every annotation in the module a string that is
never evaluated at import time, which removes this whole class of failure rather than one instance.

#1955 already named this as the complementary measure to the fix.

Current state (master ca1e60c)

• 7 modules under src/autobahn have the import; Update zope.interface dependency and add future annotations import #1959 adds 2 more.
• ruff check src/autobahn --select I002 with required-imports = ["from __future__ import annotations"]
  reports 91 files without it (39 of them test modules). Generated and vendored code is already
  excluded by the existing ruff config (src/autobahn/wamp/gen/*, src/autobahn/flatbuffers/*,
  src/autobahn/wamp/message_fbs.py).

Change

1. Enforce it, so it stays consistent without anyone remembering:

   [tool.ruff.lint]
   extend-select = ["I002"]
   
   [tool.ruff.lint.isort]
   required-imports = ["from __future__ import annotations"]

2. Apply it once, mechanically: ruff check src/autobahn --select I002 --fix, then ruff format
   on the touched files. I002 inserts the import without the blank line after it, which the
   formatter adds.

Is it safe?

Trial on a scratch copy of master, CPython 3.14: the import inserted into all 91 files, then both
suites run:

• just test-twisted cpy314: PASSED (skips=32, successes=325)
• just test-asyncio cpy314: 275 passed, 6 skipped
• test_import_all.py: all passed

Why it should be safe in general:

• The only place autobahn reads annotations at runtime is the check_types type check
  (ApplicationSession.type_check in autobahn/wamp/protocol.py). It reads the annotations of the
  user's registered functions, not of autobahn's own modules, so this change does not affect
  it. (Related, separate issue below.)
• No name becomes unavailable: every name used in an annotation today is imported at runtime,
  because the annotation is currently evaluated. Code that later calls typing.get_type_hints()
  on autobahn classes resolves the strings against the same module globals.
• On Python 3.14, annotation evaluation is deferred natively (PEP 649 / PEP 749) and the
  __future__ import keeps working. When autobahn's minimum becomes 3.14, the import can be
  dropped again; the ruff setting then goes with it.

Also to check in the PR: the documentation build (Sphinx autodoc renders the string annotations).

Out of scope

• The pre-existing ruff format drift: with the current floating ruff (0.16.10),
  ruff format --check src/autobahn already reports 42 files on master. That is a separate,
  tool-drift matter; this change should only format the lines it touches.
• Related latent bug, for its own issue: type_check uses func.__annotations__ directly and
  compares with isinstance(value, annotation). In a user module that itself uses
  from __future__ import annotations (or any string annotation), those are strings, and the check
  fails. It should resolve them, e.g. via typing.get_type_hints(func) or
  inspect.get_annotations(func, eval_str=True).

Acceptance

• Every module under src/autobahn (except generated/vendored code) starts with
  from __future__ import annotations.
• ruff enforces it (I002), so a new module without it fails just check-format.
• Twisted and asyncio test suites, the import smoke tests (including the lowest-direct job from
  Update zope.interface dependency and add future annotations import #1959) and the docs build are green.

Note: This issue was drafted with AI assistance (Claude Code).