scripts/pypackage.py: fix manylinux builds that need python3.
scripts/pypackage.py: fix manylinux builds that need python3.

The manylinux docker container does not have a python3 (though it does have
python3.8, python3.9 etc) which can be required by some builds, so we now
always create a softlink inside the container.

This fixes build of Extract when creating MuPDF wheels.

Also fixed some issues caused by changes to jlib.Arg command-line parsing.

ArtifexSoftware/mupdf,ccxvii/mupdf,ccxvii/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ccxvii/mupdf,ccxvii/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ccxvii/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,ccxvii/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf