build: allow filelock 4.x

filelock 3.x is the only line the current cap permits, and three CVEs
(CVE-2026-112940, CVE-2026-112913, CVE-2026-112907) are fixed only in
4.2.1+. The cap means every downstream that depends on litellm is held
on a vulnerable filelock with no way out, since pip has no override.

litellm uses FileLock(path, timeout=..., mode=...) and Timeout, in
litellm/proxy/client/cli/commands/configure_profiles.py and
claude_settings.py. Both keyword arguments still exist in 4.2.4 and
Timeout is still exported, so the two call sites are unaffected. The
CVEs are in the SQLite-backed ReadWriteLock / AsyncReadWriteLock, which
litellm does not use.

<5 rather than a floor bump because litellm supports Python 3.10 and
filelock 4.x requires >=3.11: widening the cap lets 3.11+ resolve 4.x
while 3.10 keeps a 3.x filelock automatically, through filelock's own
requires_python.

pyproject.toml

@@ -15,7 +15,7 @@ dependencies = [
     # When changing a floor, verify it installs + imports on every supported
     # Python with: `uv pip install --resolution=lowest-direct .`
     "fastuuid>=0.14.0,<1.0; python_version < '3.15'",
-    "filelock>=3.16.1,<4.0",
+    "filelock>=3.16.1,<5",
     "httpx[http2]>=0.28.0,<1.0",
     "openai>=2.20.0,<4.0.0",
     "python-dotenv>=1.0.0,<2.0",