Fix setup.py and pyproject.toml

[RMeli]

• Most of the information is redundant
• pip install -e .[openbabel] and similar do not work (optional dependencies)
• Trying to add optional dependencies to pyproject.toml results in flit_core.config.ConfigError: Use [project] table for metadata or [tool.flit.metadata], not both.

Deal with optional dependencies:

• pytest for tests
• requests (for large tests)
• rdkit or openbabel

[RMeli]

Closed by #152.