Fixes the false positive reported in
https://github.com/github/codeql/issues/18910
Adds a new `Annotation` class (subclass of `Expr`) which encompasses all
possible kinds of annotations in Python.
Using this, we look for string literals which are part of an annotation,
and which have the same content as the name of a (potentially) unused
global variable, and in that case we do not produce an alert.
In future, we may want to support inspecting such string literals more
deeply (e.g. to support stuff like "list[unused_var]"), but I think for
now this level of support is sufficient.
Adds two test cases having to do with type annotations. The first one
demonstrates that type annotations (even if they are never executed by
the Python interpreter) count as uses for the purposes of the unused
variable query. The second one demonstrates that this is _not_ the case
if all such uses are inside strings (i.e. forward declarations), as we
do not currently inspect the content of these strings.
This is only a temporary fix, as indicated by the TODO comment.
The real underlying issue is the fact that `isUnused` is defined in
terms of the underlying SSA variables (as these are only created
for variables that are actually used), and the fact that annotated
assignments are always considered to redefine their targets, which may
not actually be the case.
Thus, the correct fix would be to change the extractor to _disregard_
mere type annotations for the purposes of figuring out whether an
SSA variable should be created or not.
However, in the short term the present fix is likely sufficient.
