Drop unused dependencies

[kwabenantim]

Summary

Some dependencies are no longer used directly due to changes in the code over time. Some of these are now only transitively used by other dependencies, which would pull them in if they weren't listed as dependencies here.

It would simplify dependency management if these were pruned:

• py: no longer used.
• decorator: no longer used.
• six: no longer used
• importlib-metadata: no longer used. Replaced by importlib.metadata in the standard library.
• zipp: no longer used (only needed by importlib-metadata above).
• packaging: no longer used (only needed by importlib-metadata above).
• isodate: not used directly. Included transitively via rdflib.
• pyparsing: not used directly. Included transitively via rdflib.
• MarkupSafe: not used directly. Included transitively via Jinja2.
• mpmath: not used directly. Included transitively via sympy.
• lxml: not used directly. Included transitively via cellmlmanip (and rdflib).
• networkx: not used directly. Included transitively via cellmlmanip (and rdflib).