Drop spurious multhreading test

Per https://github.com/msgpack/msgpack-python/pull/641#discussion_r2315489310, sharing more than one `Packer` object is not supported.

.github/workflows/test.yml

@@ -29,8 +29,6 @@ jobs:
       - name: Prepare
         shell: bash
         run: |
-          # TODO: Workaround for Windows tests failing when upgrading `pip` with exit code 1
-          # pip install -U pip
           pip install -r requirements.txt pytest
 
       - name: Install pytest-run-parallel under free-threading

msgpack/_cmsgpack.pyx

@@ -1,5 +1,5 @@
 # coding: utf-8
-#cython: embedsignature=True, c_string_encoding=ascii, language_level=3, freethreading_compatible=True
+# cython: embedsignature=True, c_string_encoding=ascii, language_level=3, freethreading_compatible=True
 from cpython.datetime cimport import_datetime, datetime_new
 import_datetime()