4.1.5->4.2.1, the appimage grew 54M->66M. This change shrinks it back to 58M.
```
$ ls -lah
total 224M
drwxrwxr-x 5 user user 4.0K Mar 27 18:18 .
drwxrwxr-x 16 user user 4.0K Mar 23 16:02 ..
-rwxrw-r-- 1 user user 48M Dec 18 2020 electrum-4.0.9-x86_64.AppImage
-rwxrw-r-- 1 user user 54M Jan 19 14:25 electrum-4.1.5-x86_64.AppImage
-rwxr-xr-x 1 user user 58M Mar 27 18:12 electrum-4.2.1-dirty-x86_64.AppImage
-rwxrw-r-- 1 user user 66M Mar 27 15:00 electrum-4.2.1-x86_64.AppImage
```
I've used the great `ncdu` tool to investigate file sizes.
```
$ du squashfs-root-415/usr/lib/python3.7/ --max-depth=1 | sort -nr | head -n8
154608 squashfs-root-415/usr/lib/python3.7/
138864 squashfs-root-415/usr/lib/python3.7/site-packages
4720 squashfs-root-415/usr/lib/python3.7/lib-dynload
1744 squashfs-root-415/usr/lib/python3.7/encodings
664 squashfs-root-415/usr/lib/python3.7/pydoc_data
460 squashfs-root-415/usr/lib/python3.7/distutils
460 squashfs-root-415/usr/lib/python3.7/asyncio
436 squashfs-root-415/usr/lib/python3.7/email
$ du squashfs-root-421/usr/lib/python3.9/ --max-depth=1 | sort -nr | head -n8
194088 squashfs-root-421/usr/lib/python3.9/
143512 squashfs-root-421/usr/lib/python3.9/site-packages
33824 squashfs-root-421/usr/lib/python3.9/config-3.9-x86_64-linux-gnu
5244 squashfs-root-421/usr/lib/python3.9/lib-dynload
1720 squashfs-root-421/usr/lib/python3.9/encodings
696 squashfs-root-421/usr/lib/python3.9/pydoc_data
520 squashfs-root-421/usr/lib/python3.9/asyncio
464 squashfs-root-421/usr/lib/python3.9/distutils
```
We should delete `usr/lib/python3.9/config-3.9-x86_64-linux-gnu/` (which is 33M unpacked)
With py3.7 (electrum 4.1.5), this folder was named `config-3.7m-x86_64-linux-gnu`,
presumably because the default config to compile py3.7 was `--with-pymalloc`,
but maybe it is not for py3.9... ? not sure. (see https://peps.python.org/pep-3149/ )