Add section on supported Python versions

[jarrodmillman]

It would be nice to add the option to include supported Python versions from the classifiers. For example, see

• https://networkx.org/documentation/stable/release/release_3.2.1.html

Maybe a flag?

[lagru]

Easy to implement now that we got the parsing of pyproject.toml already in-place.

However, this seems very similar to what was discussed in #18. In #18 (review) (ff) @bsipocz and you are not convinced that listing minimal dependencies is necessary. What makes listing supported Python versions more important than say NumPy or the full set of dependencies?

To be clear I don't mind including this as feature at all. I'd make it an optional feature. If we limit the scope to parsing pyproject.toml and agree on formatting details this will be a simple feature to add and maintain.

[lagru]

Also some overlap with #16.

[lagru]

After talking @bsipocz and @jarrodmillman, current consensus is:

• Just grab the minimal Python version and runtime dependencies from the pyproject.toml if present and paste into its own section. Including the Python version, and dependencies should be two independent configuration options, so projects can opt-in / opt-out.
• Do the same for optional dependencies which are named in a third config option.
• Long-term we might want a lightweight plugin-system or at least some Python-scriptable customization. Though, I'd argue to wait until we have at least some feature requests or ideas to know what such a system would have to support. :)