Updates the Python extractor to depend on version 0.24.7 of tree-sitter
(and 0.12.0 of tree-sitter-graph).

A few changes were needed in order to make the code build and run after
updating the dependencies:

- In `main.rs`, the `Language` parameter is now passed as a reference.
- In `python.tsg`, many queries had captures that were not actually used
in the body of the stanza. This is no longer allowed (unless the
captures start with an underscore), as it may indicate an error. To fix
this, I added underscores in the appropriate places (and verified that
none of these unused captures were in fact bugs).

It seems that with a newer version of tree-sitter, we no longer parse
the (not actually valid!) syntax `Spam[**P2]` as if the `**` is an
exponentiation operation (with a missing left operand).

Rather than relying on matching arbitrary nodes inside tree-sitter-graph
and then checking whether they are of type ERROR or MISSING (which seems
to have stopped working in later versions of tree-sitter), we now
explicitly go through the tree-sitter tree, locating all of the error
and missing nodes along the way. We then add these on to the graph
output in the same format as was previously produced by
tree-sitter-graph.

Note that it's very likely that some of the syntax errors will move
around a bit as a consequence of this change. In general, we don't
expect syntax errors to have stable locations, as small changes in the
grammar can cause an error to appear in a different position, even if
the underlying (erroneous) code has not changed.