EL8 builds are failing because hack/markdown-preprocess needs python3
which AFAICT isn't included by default in EL8 build environments.
This commit also includes an additional `[CI:COPR]` mode which is
currently runs the same tests as `[CI:DOCS]` but could differ in future.
Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>