snap contains python 3.8 and 3.9 libs

while the folder is of course not needed i don't feel it is a trouble maker.
as far as i can see some python3-* packages place some python3.9 files there
e.g.
```
user@user-virtual-machine:~/duplicity-git/build/duplicity-0.8.23.dev9$ apt-file find /usr/lib/python3.9/distutils/archive_util.py
python3-distutils: /usr/lib/python3.9/distutils/archive_util.py
```
or
```
user@user-virtual-machine:~/duplicity-git/build/duplicity-0.8.23.dev9$ apt-file find /usr/lib/python3.9/lib2to3/btm_matcher.py
python3-lib2to3: /usr/lib/python3.9/lib2to3/btm_matcher.py
```

but for the peace of your mind we can simply just remove it from the snap package. i'll add a line in the yaml.

We had the multi-lib problem that cause the dropbox msgpack problem I reported a while back. I'm going to be adamant and say that a snap with multiple lib versions is a very strong potential problem. It just means that something is coming into the snap at the wrong version and that should not be. It may be that we need to find a way to version apt packages as well as python packages.

this is "fixed" with merge request https://gitlab.com/duplicity/duplicity/-/merge_requests/83

Thanks @edso!

To the SnapCraft team: This is still a bug. @edso has implemented a workaround, but we still do not know what the side effects of this will be. It would be best if we had a clean build to begin with, and not have to chase down something that could cause https://bugs.launchpad.net/snapcraft/+bug/1956157 to rear its ugly head again.