pdb: "until" does not step over a list comprehension with a breakpoint set on it

Bug report

When a breakpoint is set on a line that holds a list comprehension (or any
single-line loop), the until command stops on that same line again on every
iteration instead of moving past it.

Reproduction:

# repro.py
def f():
    lst = [i for i in range(10)]
    for i in lst: pass

f()

$ python -m pdb repro.py
> repro.py(1)<module>()
-> def f():
(Pdb) break 2
Breakpoint 1 at repro.py:2
(Pdb) continue
> repro.py(2)f()
-> lst = [i for i in range(10)]
(Pdb) until
> repro.py(2)f()
-> lst = [i for i in range(10)]
(Pdb) until
> repro.py(2)f()
-> lst = [i for i in range(10)]

until keeps stopping on line 2, once per comprehension element.

Expected: until moves past the comprehension to the next line, the way next
and step already do (gh-136057, gh-136160). This also matches gdb, which
pdb's until is borrowed from: there, until steps past a single-line loop
to the next line even with a breakpoint set on it, instead of stopping on each
iteration.

I have a small fix for this and can open a PR.

Linked PRs

• gh-152374

Start with the pdb entry point and reproduce the behavior using repro.py, setting a breakpoint on the list-comprehension line and running until repeatedly. Compare until with next and step as described in the issue. Done means until advances to the next line rather than stopping on each comprehension element; a linked PR is already recorded.