Update ebuild generator to support the latest systems

[gentoo90]

• fix sporadical empty *.ebuild and metadata.xml files
• EAPI6 is dead, so switch to EAPI8
• python 3.5 and 3.6 are dead, so switch to 3.10 and 3.11
• fix dependencies of the packages by evaluating conditions
• update syntax of applying patches
• add buildtool_export and build_export to the dependencies

[nuclearsandwich]

@gentoo90 would you be willing to update the GitHub actions-based CI so that we can see test feedback on these changes. If the CI will pass we'll merge these changes.

[gentoo90]

@nuclearsandwich, I've updated Ubuntu and Python versions in the CI config. Is there anything else I need to do?

[robwoolley]

Thanks for submitting these upgrades for the Gentoo support. I am not a Gentoo user, myself. Do you know if these changes are still needed given the other Gentoo fixes from @allenh1 that recently were merged?