fix: invalid SPDX license ID in project metadata

The project was using a non-existent SPDX license ID ("BSD") which lead Poetry to mark the project under a proprietary license (fallback).

For example, this can be observed at https://pypi.org/pypi/pytest-celery/1.1.3/json, where we can see the following classifier:

```
License :: Other/Proprietary License
```

Explanation: when poetry doesn't know the license (unable to match the license against the SPDX license ID list[^1]), it falls back to "Proprietary"[^2][^3].

This can cause tools checking for license compliance to mistakenly flag the project as non-compliant.

[^1]: https://spdx.org/licenses/
[^2]: https://github.com/python-poetry/poetry-core/blob/5d3abc51bb765d825f3162f34595d853b249a8eb/tests/spdx/test_license.py#L44-L47
[^3]: https://github.com/python-poetry/poetry-core/blob/ab1bdf32fbe283c3e03ea77cf55b008819b6549e/src/poetry/core/spdx/license.py#L156-L160

pyproject.toml

@@ -60,7 +60,7 @@ classifiers = [
 
 description = "Pytest plugin for Celery"
 homepage = "https://github.com/celery/pytest-celery"
-license = "BSD"
+license = "BSD-3-Clause"
 name = "pytest-celery"
 version = "1.1.3"
 readme = "README.rst"