In Pdb, when successfully clear breakpoints, the Pdb should output a message:"Deleted XXX", but when breakpoints are cleared by filename:lineno, the message can't be output.

I think it's related to the following code.

```python
pdb.py:

def do_clear(self, arg):
    ...
  
    if ':' in arg:
        # Make sure it works for "clear C:\foo\bar.py:12"
        i = arg.rfind(':')
        filename = arg[:i]
        arg = arg[i+1:]
        try:
            lineno = int(arg)
        except ValueError:
            err = "Invalid line number (%s)" % arg
        else:
            bplist = self.get_breaks(filename, lineno)
            err = self.clear_break(filename, lineno)
        if err:
            self.error(err)
        else:
            for bp in bplist:
                self.message('Deleted %s' % bp)
        return
```
self.get_breaks is called to get the breakpoints to be deleted,  the result is in bplist. self.clear_break is called to delete the breakpoints in bplist. Each element in bplist is a reference to a Breakpoint object, so when all Breakpoint objects are removed, the bplist will become an empty list when self.clear_break is called, so pdb can't output the prompt message.

It can be simply fixed by following code:
```python
    bplist = self.get_breaks(filename, lineno)[:]
```

Please provide instructions how to reproduce the bug. The patch would need a unit test as well.

Accroding to my test, it appears in python 3.8.0 and the latest 3.10 master branch, and I think other versions also have this bug.

The steps to reproduce the bug:

1. start pdb: python -m pdb foo.py

2. set a breakpoint on any line of the file:

(Pdb) b 2
Breakpoint 1 at foo.py:2
(Pdb) b
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at foo.py:2

3. when clear the breakpoint using breakpoint number, it will get a output("Deleted breakpoint 1 at ..."):

(Pdb) cl 1
Deleted breakpoint 1 at foo.py:2

4. set another breakpoint:

(Pdb) b 3
Breakpoint 2 at foo.py:3

5. if breakpoint is cleared using (filename:lineno), it gets nothing:

(Pdb)cl foo.py:2
(Pdb)

I found a typo in the reproduce step. I'm going to fix it here.

The unit test is updated in PR. Now it has some conflicts with the master branch, and I will resolve them soon.

The steps to reproduce the bug:

Assume the file to be debugged is foo.py(You can use any file you want).

1. start pdb: python -m pdb foo.py

2. set a breakpoint on any line of the file:

(Pdb) b 2
Breakpoint 1 at foo.py:2
(Pdb) b
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at foo.py:2

3. when clear the breakpoint using breakpoint number, it will get a output("Deleted breakpoint 1 at ..."):

(Pdb) cl 1
Deleted breakpoint 1 at foo.py:2

4. set another breakpoint:

(Pdb) b 3
Breakpoint 2 at foo.py:3

5. if breakpoint is cleared using (filename:lineno), it gets nothing:

(Pdb)cl foo.py:3
(Pdb)

Your fix looks correct - the problem is that the list of breakpoints gets emptied by the clear_break call, and you copy the list so that it can be echoed later if clear_break does not err.

I made some polishing comments on the PR, and it also need to be rebased because there is not a merge conflict on test_pdb.

New changeset 4cb6ba14325cff98589c2660d1d2c65f4aacfee4 by huzhaojie in branch 'main':
bpo-43318: Fix a bug where pdb does not always echo cleared breakpoints (GH-24646)
https://github.com/python/cpython/commit/4cb6ba14325cff98589c2660d1d2c65f4aacfee4

New changeset 9c0180ae7761b352116a2528aae61eea10e31045 by Miss Islington (bot) in branch '3.10':
bpo-43318: Fix a bug where pdb does not always echo cleared breakpoints (GH-24646) (GH-26674)
https://github.com/python/cpython/commit/9c0180ae7761b352116a2528aae61eea10e31045

New changeset 6df926f1c46eb6db7b5dcd0227c6b532c78525c9 by Miss Islington (bot) in branch '3.9':
bpo-43318: Fix a bug where pdb does not always echo cleared breakpoints (GH-24646) (GH-26675)
https://github.com/python/cpython/commit/6df926f1c46eb6db7b5dcd0227c6b532c78525c9