Declare license as SPDX identifier in metadata

Fixes warning seen during packaging operations

```
➜  mitogen git:(master) ✗ uv build --sdist
Building source distribution...
...
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: BSD License

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
  self._finalize_license_expression()
running egg_info
...
```

docs/changelog.rst

@@ -24,6 +24,7 @@ In progress (unreleased)
 * :gh:issue:`1266` Import cleanups
 * :gh:issue:`1266` :mod:`ansible_mitogen`: De-duplicate sys.path manipulations
 * :gh:issue:`1344` Correct SPDX license declarations
+* :gh:issue:`1344` Declare BSD-3-Clause SPDX license in package metadata
 
 
 v0.3.29 (2025-09-18)

setup.py

@@ -79,7 +79,7 @@ setup(
     long_description = long_description(),
     long_description_content_type='text/markdown',
     author = 'David Wilson',
-    license = 'New BSD',
+    license = 'BSD-3-Clause',
     url = 'https://github.com/mitogen-hq/mitogen/',
     packages = find_packages(exclude=['tests', 'examples']),
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*',
@@ -88,7 +88,6 @@ setup(
         'Environment :: Console',
         'Framework :: Ansible',
         'Intended Audience :: System Administrators',
-        'License :: OSI Approved :: BSD License',
         'Operating System :: MacOS :: MacOS X',
         'Operating System :: POSIX',
         'Programming Language :: Python',