dist: fix spec file issues

[bmr-cymru]

Suggested improvements to the snapm.spec file from the Fedora Review Bug:

• Drop centos/rhel special casing and build everything with %pyproject_wheel/%pyproject_install
• Use %pytest -- --log-level=debug instead of directly calling pytest-3
• Switch to dynamic BuildRequires

[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/snapshotmanager-snapm-190
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.