Replace the deprecated `bdist_rpm` method of creating RPMs for
Dangerzone. Instead, update our `install/linux/build-rpm.py` script, to
build Dangerzone RPMs using our SPEC file under
`install/linux/dangerzone.spec`. The script now essentially creates a
source distribution (sdist) using `poetry build`, and then uses
`rpmbuild` to create binary and source RPMs.
Fixes#298
Introduce a SPEC file that can be used to create an RPM from a Python
source distribution. Some notable features of this SPEC file follow:
1. We can use this SPEC file to create both regular RPM packages and
ones targeted for Qubes.
2. It has a post installation script that removes stale .egg-info
directories, which previously caused issues to our users.
3. It automatically creates a changelog from our Git logs, which differs
from the actual CHANGELOG.md.
4. It folloes the latest Fedora guidelines (as of writing this) for
packaging Python projects.
Fixes#514
