b0.71 backport: Delete the spurious pbench3-devel RPM dependency

Fixes distributed-system-analysis#2798

This is a backport of distributed-system-analysis#2799 to b0.71.

AFAICT, the python3-devel dependency (which only exists for RHEL7) is
spurious and in fact harmful: there seems to be no python3-devel
package for RHEL7.9, so installation fails with an unsatisfied
dependency, whereas an RPM built without it installs without
problems.

agent/rpm/pbench-agent.spec.j2

@@ -18,7 +18,7 @@ Requires:  ansible-core
 Requires:  python3, python3-pip
 # For RHEL boxen we need the python development environment in order for the
 # pip3 installs for some modules to be successful.
-Requires:  gcc python3-devel
+Requires:  gcc
 %endif
 
 %if 0%{?rhel} == 8