pip generator does not collect build dependencies

[theCapypara]

flatpak-builder version

1.2.3

Linux distribution and version

Arch Linux

Affected flatpak-builder tool

pip/flatpak-pip-generator

flatpak-builder tool cli args

No response

Source repository URL

No response

Flatpak-builder manifest URL

No response

Description

I was surprised to not find an issue for this yet. Please close this if I just missed one.

Currently more and more Python packages require build isolation to be built, due to them having build dependencies. This currently doesn't work properly because of the pip generator not supporting collecting these build dependencies. As such one has to manually collect and insert them.

This is seemingly blocked by pypa/pip#7863.

#296 would also eliminate/side-step this issue and other potential headache.

This is probably the cause for at least some of these issues: #47, #90, #100, #218, #255.
Basically (almost?) any package that requires build isolation will not work because of this. Since build isolation is the recommended setup for new packages, this will become an ever-increasing issue.