Proxypatternpool

- Enable _PyPy 3.11_ GitHub CI.
- Enable _Python 3.14t_ on GitHub CI, including a coverage issue workaround for now.
- Add a _nogil_ test.
- Improve documentation.

- Trace double return objects.
- Enable _Python 3.13t_ CI with and without GIL.
- Improve tests.
- Add convenient `dev` and `clean.dev` make targets.

- Enable _Python 3.14_ CI.
- Improve documentation.
- Improve `yield` pattern.

- Use `ProxyException` instead of `Exception` in one place.
- Activate _Pypy 3.10_ and _Python 3.13_ on GitHub CI, with some dependency exclusions.

- Add delayed pool parameter initialization with `_set_pool`.

- Improve documentation.
- Add workaround against `Flask --debug` initial reload, triggered by setting
environment variable `PPP_WERKZEUG_WORKAROUND`.