gh-140790: pdb: Initialize instance variables in Pdb.__init__

[Carreau]

Initialize lineno, stack, curindex, curframe,
currentbp, and _user_requested_quit attributes in
Pdb.__init__ instead of at first use. This ensures these attributes are always defined and allows
removal of defensive getattr/hasattr checks
throughout the codebase.

In particular for a custom PDB subclass in
IPython, the need to use a getattr has performance implications, and while we could set it our __init__ it was easy to send a more global patch to CPython.

Should fix gh-140790

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 CentOS9 NoGIL Refleaks 3.x (tier-1) has failed when building commit 4885ecf.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1610/builds/2417) and take a look at the build logs.
4. Check if the failure is related to this commit (4885ecf) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1610/builds/2417

Failed tests:

• test_free_threading

Test leaking resources:

• test_free_threading: file descriptors

Summary of the results of the build (if available):

==

Click to see traceback logs

remote: Enumerating objects: 9, done.        
remote: Counting objects:  12% (1/8)        
remote: Counting objects:  25% (2/8)        
remote: Counting objects:  37% (3/8)        
remote: Counting objects:  50% (4/8)        
remote: Counting objects:  62% (5/8)        
remote: Counting objects:  75% (6/8)        
remote: Counting objects:  87% (7/8)        
remote: Counting objects: 100% (8/8)        
remote: Counting objects: 100% (8/8), done.        
remote: Compressing objects:  12% (1/8)        
remote: Compressing objects:  25% (2/8)        
remote: Compressing objects:  37% (3/8)        
remote: Compressing objects:  50% (4/8)        
remote: Compressing objects:  62% (5/8)        
remote: Compressing objects:  75% (6/8)        
remote: Compressing objects:  87% (7/8)        
remote: Compressing objects: 100% (8/8)        
remote: Compressing objects: 100% (8/8), done.        
remote: Total 9 (delta 0), reused 0 (delta 0), pack-reused 1 (from 1)        
From https://github.com/python/cpython
 * branch                    main       -> FETCH_HEAD
Note: switching to '4885ecfbda4cc792691e5d488ef6cb09727eb417'.

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Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 4885ecfbda4 gh-140790: pdb: Initialize instance variables in Pdb.__init__ (#140791)
Switched to and reset branch 'main'

configure: WARNING: no system libmpdec found; falling back to pure-Python version for the decimal module

make: *** [Makefile:2493: buildbottest] Error 2

[gaogaotiantian]

The buildbot alert seems to be irrelevant.

[Carreau]

Thank you for the review and the merge!