Add support for PEP 723 -- Inline script metadata

[jherland]

See https://peps.python.org/pep-0723/ for details.

One complication is that all .py (and .ipynb?) files will now have to be parsed for these dependency declarations (as well as for import statements). This might create some challenges in our overall "flow" of imports vs. declared_deps.

[jherland]

Thinking more about this: If a code file contains PEP 723 metadata/dependencies, does it make sense to analyze that file separately from the rest of the project?

• It does not makes sense for a dependency declared in a PEP 723 section to be matched with an import in a different file. Rather, the dependency declaration in that code file should be considered "local" to that file, and not be matched with any import outside that file.
• I suspect that it does not make sense for an import in a code file with a PEP 723 section to be satisfied by a dependency declaration from outside that file (e.g. from a nearby requirements.txt). The existence of a PEP 723 section makes it clear(?) that this file performs its own dependency declarations, and should thus be considered as a separate "project" in its own right?

If these are both true, then I suspect that the easiest way to handle PEP 723-using code files is to run a separate analysis (even going as far as creating a separate Analysis object) for these files. This "sub-analysis" would be separate from the overall project analysis, and would be ~equivalent to running FawltyDeps separately on that path alone (passing something like --code <path> --deps <path>, still unsure what to do about --pyenv...)

Care must be taken when integrating the result of this analysis into the overall analysis/report, for example, consider a PEP 723-using code file with an undeclared dependency, and a requirements.txt that happens to declare the same dependency (but is actually unrelated due to the second point above). Without noting this in the output, we would simply get something like:

These imports appear to be undeclared dependencies:
- 'foo' 
    imported at:
      a_file_using_pep723.py

These dependencies appear to be unused (i.e. not imported):
- 'foo'
    declared in:
      requirements.txt

which is really confusing until you understand all the intricacies of PEP 723...

Something like this would already be much better

These imports appear to be undeclared dependencies:
- 'foo' 
    imported at:
      a_file_using_pep723.py
        (This file uses inline script metadata, and is therefore analyzed stand-alone)
      

These dependencies appear to be unused (i.e. not imported):
- 'foo'
    declared in:
      requirements.txt