bpo-38054: enhance pdb.set_trace() to run when the specified condition is true

[alex-ulnv]

• Added a new key-word argument trigger to pdb.set_trace() method. If specified, set_trace() will execute only when the condition is true.
• Added a unit test for the trigger kwarg
• Updated documentation

https://bugs.python.org/issue38054

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept your contribution by verifying you have signed the PSF contributor agreement (CLA).

Our records indicate we have not received your CLA. For legal reasons we need you to sign this before we can look at your contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

If you have recently signed the CLA, please wait at least one business day
before our records are updated.

You can check yourself to see if the CLA has been received.

Thanks again for your contribution, we look forward to reviewing it!

[zware]

As mentioned in the issue, this use-case is better served by conditional expressions or an and short-circuit. Thanks for the idea, though!