Drop the deprecated license classifier

[Kayvan-Zahiri]

Closes #996.

setuptools 77 deprecated License :: OSI Approved :: * in favour of the PEP 639
SPDX expression. setup.py already carries license="Apache-2.0", so the
classifier is the only thing producing the warning.

Built metadata before and after, from setup.py egg_info:

  before                                                    after
  Metadata-Version: 2.4                                     Metadata-Version: 2.4
  License: Apache-2.0                                       License: Apache-2.0
  Classifier: License :: OSI Approved :: Apache Software    (removed)
  License-File: LICENSE                                     License-File: LICENSE

Nothing is lost: the SPDX expression and the bundled LICENSE both survive.

build-system.requires moves to setuptools>=77.0.3, which is the release that
reads license as an SPDX expression. Without the floor, an older setuptools
would treat it as free text and the classifier removal would cost real metadata.

Also removed tests_require, which the same build reports as
UserWarning: Unknown distribution option: 'tests_require' and ignores. Happy to
split that into its own PR if you would rather keep this to the license change.

Verified with setuptools 84.0.0: both warnings are gone and pytest tests/unit_tests is unchanged at 1444 passing.

[joe-clickhouse]

Thanks. I'm good with this, but will reqeust one fix before merge.

license="Apache-2.0" is not read as SPDX by any setuptools release. It still emits the deprecated free text License: field, which your before and after output shows. Setuptools writes License-Expression: only from the license_expression attribute.

Please make these changes:

1. In setup.py, replace license="Apache-2.0" with license_expression="Apache-2.0"
2. Keep the classifier deletion and the setuptools>=77.0.3 floor. Both are required with this change
3. Reword the changelog entry and PR description. Setuptools does not reinterpret license= as an SPDX expression. The accurate statement is that the package now declares its license with the PEP 639 license_expression field.

The result should build with no warnings and emit License-Expression: Apache-2.0 plus License-File: LICENSE.