REQUIREMENTS: Expanded the bandwith of allowed p4p versions

When trying to run pip install -r requirements-epics.txt on my RHEL8
machine, I found that the fixed requirement of p4p=4.0.0 caused
problems, since some binaries could not be built. Researching the
problem showed that 4.0.0 is almost three years old. Trying to install
the newest version 4.2.0 succeded. Therefore, I'd like to make the
requirements less strict.

Change-Id: I13b55568e7624041d0de65c2c7c476f68dae93d5
Reviewed-on: https://forge.frm2.tum.de/review/c/frm2/nicos/nicos/+/35004
Reviewed-by: Enrico Faulhaber <[email protected]>
Reviewed-by: Jens Krueger <[email protected]>
Tested-by: Jenkins Automated Tests <[email protected]>

requirements-epics.txt

@@ -1,3 +1,3 @@
 caproto>=0.8.1;platform_machine!='arm64'
-p4p==4.0.0;platform_machine!='arm64'
+p4p>=4.0.0,<=4.2.0;platform_machine!='arm64'
 pyepics>=3.2.4;platform_machine!='arm64'