Add no-GIL interpreter support - #641

On Windows, attempting to upgrade `pip` within CI always yields this:
```
Run pip install -U pip
Requirement already satisfied: pip in c:\hostedtoolcache\windows\python\3.10.11\x64\lib\site-packages (25.1.1)
Collecting pip
  Downloading pip-25.2-py3-none-any.whl.metadata (4.7 kB)
Downloading pip-25.2-py3-none-any.whl (1.8 MB)
   ---------------------------------------- 1.8/1.8 MB 48.1 MB/s eta 0:00:00

Notice:  A new release of pip is available: 25.1.1 -> 25.2
Notice:  To update, run: python.exe -m pip install --upgrade pip
ERROR: To modify pip, please run the following command:
C:\hostedtoolcache\windows\Python\3.10.11\x64\python.exe -m pip install -U pip
Error: Process completed with exit code 1.
```

Co-authored-by: Frank Dana <ferdnyc@gmail.com>

Per msgpack#641 (comment), sharing more than one `Packer` object is not supported.