Add `beautifulsoup4` to the dependency list
# What does this PR do?

Add `beautifulsoup4` to the dependency list - to enable the tests `MarkupLMFeatureExtractionTest` and `MarkupLMProcessorIntegrationTests`

On CircleCI, we now can have the following tested (those have `@require_bs4`)
```bash
PASSED tests/models/markuplm/test_feature_extraction_markuplm.py::MarkupLMFeatureExtractionTest::test_call
PASSED tests/models/markuplm/test_feature_extraction_markuplm.py::MarkupLMFeatureExtractionTest::test_feat_extract_from_and_save_pretrained
PASSED tests/models/markuplm/test_feature_extraction_markuplm.py::MarkupLMFeatureExtractionTest::test_feat_extract_to_json_file
PASSED tests/models/markuplm/test_feature_extraction_markuplm.py::MarkupLMFeatureExtractionTest::test_feat_extract_to_json_string
PASSED tests/models/markuplm/test_feature_extraction_markuplm.py::MarkupLMFeatureExtractionTest::test_init_without_params
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @sgugger I think I misread your message, and end up putting the installation in the CircleCI `config.yml` and GitHub workflow files. Should I instead put `beautifulsoup4` in `extras["testing"]`? That way might be much easier.<|||||>Yes I meant to add it in `extras["testing"]`.<|||||>test failure irrelevant, merge now.