Add Python 3.15 to the CI test matrix

Mirrors how 3.14 was added in e709c83: a matrix entry in build.yml and the
corresponding env in the tox envlist. The existing `${{ matrix.py }}-dev`
suffix on setup-python already picks up the newest prerelease, so no extra
handling is needed.

No classifier is added yet. The 3.14 classifier landed once 3.14 reached
rc; 3.15 is still in beta, so this only starts testing against it rather
than declaring support.

Signed-off-by: Bernát Gábor <gaborjbernat@gmail.com>

Author: gaborbernat

.github/workflows/build.yml

@@ -21,6 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          - { "py": "3.15", "tox_env": "py315" }
           - { "py": "3.14", "tox_env": "py314" }
           - { "py": "3.13", "tox_env": "py313" }
           - { "py": "3.12", "tox_env": "py312-cov" }

tox.ini

@@ -1,5 +1,6 @@
 [tox]
 envlist =
+    py315
     py314
     py313
     py312-cov