WIP app_python3: need a test for a real child process

[aalba6675]

app_python3 needs to call PyOS_AfterFork_Child()/PyOS_AfterFork()
in real child processes.

We need a test for this: rank > 0 may not be sufficient due to
non-forking mode. This POC uses getpid().

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

@miconda @henningw : this PR is for discussion only: in KEMI Python there is an interpreter function that needs to be called after fork() in real child processes. I realise that rank > 0 may not be a completely correct test as kamailio has a non-forking mode(this is the test used in master and 5.6 to fix GH#3125). Your comments would be appreciated.

This POC uses getpid() to detect a “real” child process.

Is there a flag somewhere that a module can tell that it is running in a child process?