build: enable PyO3 generate-import-lib for Windows free-threaded wheels

Windows free-threaded Python does not expose `abiflags` in sysconfig,
so PyO3's default Windows linkage path fails with "A python 3
interpreter on Windows does not define abiflags in its sysconfig ಠ_ಠ"
when building cp31Xt wheels. Enabling the `generate-import-lib` PyO3
feature switches Windows builds to a generated import library
(provided by the `python3-dll-a` crate) that does not depend on a
fully populated sysconfig. It is a no-op on macOS and Linux and is
compatible with the existing `abi3` feature.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

Cargo.lock

@@ -40,7 +40,7 @@ tokio = { workspace = true, features = [
   "rt-multi-thread",
   "sync",
 ] }
-pyo3 = { workspace = true, features = ["extension-module"] }
+pyo3 = { workspace = true, features = ["extension-module", "generate-import-lib"] }
 pyo3-async-runtimes = { workspace = true, features = ["tokio-runtime"] }
 pyo3-log = { workspace = true }
 chrono = { workspace = true }