This is likely covered by existing/linked issues already, but wanted to leave it here nonetheless:

Given t-pdb.py:
```
import sys


def main():
    sys.stdout.write("main...\n")
    assert 0


if __name__ == "__main__":
    main()
```

Without the fix from the PR:
```
% python3.8 -m pdb -c cont t-pdb.py
main...
Traceback (most recent call last):
  File "…/pyenv/3.8.0/lib/python3.8/pdb.py", line 1702, in main
    pdb._runscript(mainpyfile)
  File "…/pyenv/3.8.0/lib/python3.8/pdb.py", line 1571, in _runscript
    self.run(statement)
  File "…/pyenv/3.8.0/lib/python3.8/bdb.py", line 587, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "…/Vcs/cpython/t-pdb.py", line 1, in <module>
    import sys
  File "…/Vcs/cpython/t-pdb.py", line 6, in main
    assert 0
AssertionError
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> …/Vcs/cpython/t-pdb.py(6)main()
-> assert 0
(Pdb)
```

With the fix:
```
% /tmp/cpython-bisect/bin/python3.8 -m pdb -c cont t-pdb.py
main...
Traceback (most recent call last):
  File "/tmp/cpython-bisect/lib/python3.8/pdb.py", line 1703, in main
    pdb._runscript(mainpyfile)
  File "/tmp/cpython-bisect/lib/python3.8/pdb.py", line 1572, in _runscript
    self.run(statement)
  File "/tmp/cpython-bisect/lib/python3.8/bdb.py", line 583, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "…/Vcs/cpython/t-pdb.py", line 10, in <module>
    main()
  File "…/Vcs/cpython/t-pdb.py", line 6, in main
    assert 0
AssertionError
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> …/Vcs/cpython/t-pdb.py(6)main()
-> assert 0
(Pdb)
```

As you can see the traceback in the fixed case contains `main()` correctly,
while it has `import sys` (the scripts first line) otherwise.

I can only repeat myself to ask for reviewing/merging https://github.com/python/cpython/pull/12419.