Error with dynamic classifiers in `pyproject.toml`

[abravalheri]

Previously reported in #3195 by @domdfcoding:

There is a separate issue where if classifiers is in dynamic, setuptools throws an error:

  File "/tmp/build-env-lbp441r9/lib/python3.8/site-packages/setuptools/dist.py", line 836, in parse_config_files
    pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
  File "/tmp/build-env-lbp441r9/lib/python3.8/site-packages/setuptools/config/pyprojecttoml.py", line 52, in apply_configuration
    config = read_configuration(filepath, True, ignore_option_errors, dist)
  File "/tmp/build-env-lbp441r9/lib/python3.8/site-packages/setuptools/config/pyprojecttoml.py", line 118, in read_configuration
    return expand_configuration(asdict, root_dir, ignore_option_errors, dist)
  File "/tmp/build-env-lbp441r9/lib/python3.8/site-packages/setuptools/config/pyprojecttoml.py", line 163, in expand_configuration
    _expand_all_dynamic(project_cfg, setuptools_cfg, package_dir, root_dir, ignore)
  File "/tmp/build-env-lbp441r9/lib/python3.8/site-packages/setuptools/config/pyprojecttoml.py", line 252, in _expand_all_dynamic
    project_cfg["classifiers"] = value.splitlines()
AttributeError: 'NoneType' object has no attribute 'splitlines'