Python: Fix generator expression locations

Our logic for detecting the first and last item in a generator
expression was faulty, sometimes matching comments as well. Because
attributes (like `_location_start`) can only be written once, this
caused `tree-sitter-graph` to get unhappy.

To fix this, we now require the first item to be an `expression`, and
the last one to be either a `for_in_clause` or an `if_clause`.
Crucially, `comment` is neither of these, and this prevents the
unfortunate overlap.

python/extractor/tests/parser/comprehensions.py

@@ -65,3 +65,10 @@ t = tuple(x for y in z)
     d for e in f if g # comment
     # comment
 ] # comment
+
+# Generator expression with comments
+(# comment
+    alpha # comment
+    for beta in gamma # comment
+    # comment
+)