Around transitions from one python minor version to another (such as 3.7 to
3.8), the current packaging can be slightly problematic - it produces a
control file that requires that the version of python3 that's installed be
exactly the one that was used on the build machine for the mercurial package,
by containing a line like:
Depends: sensible-utils, libc6 (>= 2.14), python3 (<< 3.8), python3 (>= 3.7~), python3:any (>= 3.5~)
This is because it "knows" we only built for v3.7, which is the current default
on my system. By building the native components for multiple versions, we can
make it produce a line like this, which is compatible with 3.7 AND 3.8:
Depends: sensible-utils, libc6 (>= 2.14), python3 (<< 3.9), python3 (>= 3.7~), python3:any (>= 3.5~)
This isn't *normally* required, so I'm not making it the default. For those that
receive their python3 and mercurial packages from their distro, and/or don't
have to worry about a situation where the team that manages the python3
installation isn't the same as the team that manages the mercurial installation,
this is probably not necessary.
I chose the names DEB_HG_* because DEB_* is passed through debuild
automatically (otherwise we'd have to explicitly allow the options through,
which is a nuisance), and the HG part is to make it clear that this isn't a
"standard" debian option that other packages might respect.