Add changenotes for recent .pth fixes to templates.

[freakboy3742]

This PR doesn't fix anything itself, as the fixes are all in platform templates:

• Add processing of .pth files from "app_packages". briefcase-macOS-Xcode-template#72
• Add processing of .pth files from "app_packages". briefcase-windows-VisualStudio-template#52
• Add processing of .pth files from "app_packages". briefcase-linux-flatpak-template#58
• Add processing of .pth files from "app_packages". briefcase-linux-system-template#36
• Add processing of .pth files from "app_packages". briefcase-iOS-Xcode-template#49

There is also a change on the support testbed (beeware/Python-support-testbed#118)

This should not be merged until beeware/Python-support-testbed#118 has been reviewed and merged.

It won't fix the .pth issue on Web or Android. Android requires a new Chaquopy release to fix the problem; web requires a much more complex injection process that Emscripten and pyscript don't support as of yet.

Fixes #381.
Refs #2204.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct