Add no-GIL interpreter support

Add `pytest-run-parallel` as dependency, test no-GIL interpreters in CI, and
mark Cython module as safe for freethreaded interpreters.

test/test_multithreading.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+from concurrent.futures import ThreadPoolExecutor
+from msgpack import Packer
+import threading
+
+
+def run_threaded(
+    func,
+    num_threads=8,
+    pass_count=False,
+    pass_barrier=False,
+    outer_iterations=1,
+    prepare_args=None,
+):
+    """Runs a function many times in parallel"""
+    for _ in range(outer_iterations):
+        with ThreadPoolExecutor(max_workers=num_threads) as tpe:
+            if prepare_args is None:
+                args = []
+            else:
+                args = prepare_args()
+            if pass_barrier:
+                barrier = threading.Barrier(num_threads)
+                args.append(barrier)
+            if pass_count:
+                all_args = [(func, i, *args) for i in range(num_threads)]
+            else:
+                all_args = [(func, *args) for i in range(num_threads)]
+            try:
+                futures = []
+                for arg in all_args:
+                    futures.append(tpe.submit(*arg))
+            finally:
+                if len(futures) < num_threads and pass_barrier:
+                    barrier.abort()
+            for f in futures:
+                f.result()
+
+
+def test_multithread_packing():
+    output = []
+    test_data = "abcd" * 10_000_000
+    packer = Packer()
+
+    def closure(b):
+        data = packer.pack(test_data)
+        output.append(data)
+        b.wait()
+
+    run_threaded(closure, num_threads=10, pass_barrier=True, pass_count=False)