[7.3] Move rpm to python3

[mwinter-osr]

Update RPM packages to move them all to Python 3 (instead of Python 2) with the exception of CentOS 6 which stays at Python 2.7

Tested to build on CentOS 6, 7 & 8 and Fedora 29 & 31

This is the same as #5728 for 7.3 branch

[NetDEF-CI]

Continuous Integration Result: FAILED

See below for issues.
CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-10550/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

Get source / Pull Request: Successful

Building Stage: Failed

FreeBSD 11 amd64 build: Failed (click for details)

DejaGNU Unittests (make check) failed for FreeBSD 11 amd64 build
see PyTest log at https://ci1.netdef.org/browse/FRR-FRRPULLREQ-10550/artifact/CI009BUILD/ErrorLog/log_pytests.txt
FreeBSD 11 amd64 build: config.status output from configure script can be found at https://ci1.netdef.org/browse/FRR-FRRPULLREQ-10550/artifact/CI009BUILD/config.status/config.status

Successful on other platforms

• Fedora 29 amd64 build
• FreeBSD 12 amd64 build
• Debian 10 amd64 build
• Ubuntu 16.04 amd64 build
• Ubuntu 14.04 amd64 build
• Ubuntu 16.04 i386 build
• Ubuntu 18.04 amd64 build
• Debian 8 amd64 build
• NetBSD 6 amd64 build
• CentOS 7 amd64 build
• Ubuntu 18.04 ppc64le build
• Debian 9 amd64 build
• OpenBSD 6 amd64 build
• Ubuntu 12.04 amd64 build
• NetBSD 8 amd64 build

[NetDEF-CI]

Continuous Integration Result: SUCCESSFUL

Congratulations, this patch passed basic tests

Tested-by: NetDEF / OpenSourceRouting.org CI System

CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-10550/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

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