I've modified the Sigil installer files to easily allow the creation of a Sigil-Legacy installer that should work on Windows 7sp1 (from the same Sigil codebase). The only difference is that it packages Python 3.8 instead of 3.9. If it works, it SHOULD make it trivial for me to include a legacy installer in the regular Sigil releases.
There's no guarantees how long I'll be able to keep it up, but as long as our codebase supports building with Qt5 and Python3.8, there shouldn't be any immediate problems.
If someone
can download it (from my personal google drive--be patient) and verify that it works on Windows 7, I'll add it to the 1.9.0 release (and future releases).
I thought about a lot of different options, and this is the solution I'm most comfortable with. I just couldn't bring myself to include an outsourced binary dll (or to modify Sigil's code base to build the outsourced dll) in all of Sigil's Windows installer packages.
I'm not doing an updated release (or changing the current 1.9.0 Windows installers), but starting with the next release, the "normal" installers will not work on Windows 7. The legacy installer will be your only option.
Hope that satisfies everyone.
P.S. Oh, and note that the legacy version will install as Sigil-Legacy (Control Panel) and will install to a "Sigil-Legacy" folder. It will, however, still use the same preference folder as previous versions. So all current plugins and settings should work as they did.