Replace hard-coded attribute keys

[flange-ipb]

At the moment the keys of node and edge attributes are hard-coded strings. This PR replaces all of them with symbols that are defined in the new module tucan.graph_attributes.

• Backwards compatibility is preserved: You can still use strings as attribute keys.
• (De-)Serialization: A mapping from node/edge attribute keys in the Graph data structure to atom attribute keys in TUCAN strings and vice-versa is introduced. (_SERIALIZER_NODE_ATTRIBUTE_MAPPING in tucan/serialization.py and _DESERIALIZER_NODE_ATTRIBUTE_MAPPING in tucan/parser/parser.py)
• Misc. changes:
  • in tests/io/test_molfile_v2000_reader.py: Add a test for the code that processes the charge field in the atom block of V2000 Molfiles. This is not yet covered by the corpus of test Molfiles.
  • correct type hints in the parser module

[JanCBrammer]

Looks good! Just to be sure, I've used VSCode search to find potentially unconverted attribute keys. There aren't any.