Remove rpm-py-installer, update test docs and update Dockerfiles

Remove rpm-py-installer from requirements
Check duplicite and unused packages in Dockerfiles and test-requirements
Update docs about unit tests.

Fixes: https://pagure.io/koji/issue/3868

devtools/containers/Dockerfile.f34

@@ -2,25 +2,11 @@ FROM registry.fedoraproject.org/fedora:34
 RUN \
   dnf -y update --nodocs --setopt=install_weak_deps=False && \
   dnf install -y --nodocs --setopt=install_weak_deps=False \
-  'dnf-command(download)' \
   gcc \
-  git \
-  glib2-devel \
   glibc-langpack-en \
   krb5-devel \
-  libffi-devel \
-  python3-librepo \
-  libxml2-devel \
-  make \
-  openssl-devel \
   python3-devel \
-  python3-pip \
-  python3-pytest \
-  python3-rpm \
+  python3-librepo \
   python3-tox \
-  redhat-rpm-config \
-  rpm-build \
-  rpm-devel \
-  sqlite-devel \
-  yum-utils && \
-  dnf clean all\
+  rpm-build && \
+  dnf clean all