chore: refresh PyPy CI matrix to maintained versions (#37)

## Summary

Our CI was the only PyPy line tested was `pypy-3.8`, which reached
end-of-life in October 2024 (last release December 2022). Replace it
with the actively maintained PyPy lines.

## State per upstream

Source: https://downloads.python.org/pypy/versions.json

| PyPy line | Last stable | Status |
|---|---|---|
| `pypy-3.8`  | 2022-12-29 | **EOL** |
| `pypy-3.9`  | 2024-04-24 | Effectively EOL — no releases in ~2 years |
| `pypy-3.10` | 2025-02-26 | Maintained |
| `pypy-3.11` | 2026-03-13 | Current stable |

## Changes

- **`tox.ini`** — `envlist`: `pypy38` → `pypy310, pypy311`. Same swap in
the `basepython` block.
- **`.github/workflows/github-actions.yml`** — 3 `pypy38` matrix entries
removed, 6 new entries added (`pypy310` and `pypy311`, each across
`ubuntu-latest`, `windows-latest`, `macos-latest`), mirroring the
existing CPython 3-OS pattern.

`pypy-3.9` is intentionally skipped — its last release was April 2024
with no signal of ongoing maintenance, so testing it would just inherit
the same stale-CI problem we're fixing.

`setup.py` classifiers don't enumerate PyPy minor versions (only
`Implementation :: PyPy`), so no metadata change is needed there.

The Jinja template at
`ci/templates/.github/workflows/github-actions.yml` still has a
`['py37', 'pypy37']` exclusion that's now dead code (those envs have
been gone since #32). Cleaning the template is out of scope for this PR.

Closes #36

1 files changed, 29 insertions, 12 deletions

diff --git a/.github/workflows/github-actions.yml b/.github/workflows/github-actions.yml
index 1af692a..24a45eb 100644
--- a/.github/workflows/github-actions.yml
+++ b/.github/workflows/github-actions.yml
@@ -87,22 +87,39 @@ jobs:
             toxpython: 'python3.11'
             tox_env: 'py311'
             os: 'macos-latest'
-          - name: 'pypy38 (ubuntu)'
-            python: 'pypy-3.8'
-            toxpython: 'pypy3.8'
+          - name: 'pypy310 (ubuntu)'
+            python: 'pypy-3.10'
+            toxpython: 'pypy3.10'
             python_arch: 'x64'
-            tox_env: 'pypy38'
+            tox_env: 'pypy310'
             os: 'ubuntu-latest'
-          - name: 'pypy38 (windows)'
-            python: 'pypy-3.8'
-            toxpython: 'pypy3.8'
+          - name: 'pypy310 (windows)'
+            python: 'pypy-3.10'
+            toxpython: 'pypy3.10'
             python_arch: 'x64'
-            tox_env: 'pypy38'
+            tox_env: 'pypy310'
             os: 'windows-latest'
-          - name: 'pypy38 (macos)'
-            python: 'pypy-3.8'
-            toxpython: 'pypy3.8'
-            tox_env: 'pypy38'
+          - name: 'pypy310 (macos)'
+            python: 'pypy-3.10'
+            toxpython: 'pypy3.10'
+            tox_env: 'pypy310'
+            os: 'macos-latest'
+          - name: 'pypy311 (ubuntu)'
+            python: 'pypy-3.11'
+            toxpython: 'pypy3.11'
+            python_arch: 'x64'
+            tox_env: 'pypy311'
+            os: 'ubuntu-latest'
+          - name: 'pypy311 (windows)'
+            python: 'pypy-3.11'
+            toxpython: 'pypy3.11'
+            python_arch: 'x64'
+            tox_env: 'pypy311'
+            os: 'windows-latest'
+          - name: 'pypy311 (macos)'
+            python: 'pypy-3.11'
+            toxpython: 'pypy3.11'
+            tox_env: 'pypy311'
             os: 'macos-latest'
           - name: 'py312 (ubuntu)'
             python: '3.12'