chore(deps): update greenlet requirement from <4,>=3.0 to >=3.5.4,<4 in /backend

[dependabot]

Updates the requirements on greenlet to permit the latest version.

Changelog

Sourced from greenlet's changelog.

3.5.4 (2026-07-22)

• Fix a crash (segfault) on free-threaded builds of Python 3.14 and later when the garbage collector runs while a greenlet that was started from a non-empty C-stack-reference state is active. See issue 515 <https://github.com/python-greenlet/greenlet/issues/515>_. Thanks to ddorian and Kumar Aditya.

• Fix a potential use-after-free on free-threaded builds of Python 3.14 and later when the garbage collector runs while a greenlet is suspended holding a _PyCStackRef (for example, mid attribute resolution). See issue 515 <https://github.com/python-greenlet/greenlet/issues/515>_. Thanks to ddorian and Kumar Aditya.

• Fix a deadlock on free-threaded builds when a greenlet switch happened while a PyCriticalSection was held -- for example inside asyncio's Task.__step, which holds one on the running task for the duration of the step. See PR 519 <https://github.com/python-greenlet/greenlet/pull/519/>. Thanks to ddorian and Kumar Aditya.

.. note:: Binary 3.15 wheels are now built with Python 3.15b4. These may not be compatible with earlier or later versions of 3.15. Binary 3.15 wheels of greenlet from previous releases (e.g., 3.5.3) may not be compatible with Python 3.15b4.

3.5.3 (2026-06-26)

• Fix a crash on free-threaded builds when multiple greenlets were holding a critical section on an object and the GIL for the thread was dropped. See issue 513 <https://github.com/python-greenlet/greenlet/issues/513>_. Thanks to ddorian.

3.5.2 (2026-06-17)

• The minimum supported version of Python 3.15 is now 3.15b2.
• Fix some garbage-collection related crashes on free-threaded Python 3.15. Thanks to Kumar Aditya in PR [#511](https://github.com/python-greenlet/greenlet/issues/511) <https://github.com/python-greenlet/greenlet/pull/511>_.
• Improve garbage collection of greenlets. This mostly applies to Python 3.15. Thanks to Kumar Aditya in PR [#512](https://github.com/python-greenlet/greenlet/issues/512) <https://github.com/python-greenlet/greenlet/pull/512>_.

... (truncated)

Commits

• 384be88 Preparing release 3.5.4
• bbdf57b Add note to CHANGES about versions of 3.15 binary wheels may/not be compatibl...
• 7599023 Merge pull request #517 from ddorian/issue515-c-stack-refs
• 73a0a1a Fix the suspended C-stack-ref GC test to catch its regression
• 1ff35f4 Merge branch 'master' into issue515-c-stack-refs
• 847fb82 Merge pull request #520 from dynapx/fix-test-extension-npd
• 0b471ba Merge pull request #519 from ddorian/freethread-switch-critical-section
• d55914d Simplify the CHANGES.rst entry
• 78932dc Change notes are for end users, they don't need to go into technical detail
• 39bf70c Hold the C-stack ref snapshot in a std::vector<OwnedObject>
• Additional commits viewable in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot recreate will recreate this PR, overwriting any edits that have been made to it
• @dependabot show <dependency name> ignore conditions will show all of the ignore conditions of the specified dependency
• @dependabot ignore this major version will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this minor version will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)