app_python3: fix Python 3.7 deprecation warnings

[amessina]

Pre-Submission Checklist

• 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 [Feature Request] Fix Python 2 & 3 build warnings #1748 (replace XXXX with an open issue number)

Description

Add compile-time checks for PY_VERSION_HEX >= 0x03070000

[miconda]

Thanks!

[amessina]

Thank you @miconda, I think this can go into 5.2 if that's safe enough.