Force Python 3.9 in RPM package builds

skupper-router.spec.rpkg

@@ -33,6 +33,9 @@
 %global libwebsockets_minimum_version 3.0.1
 %global libnghttp2_minimum_version 1.33.0
 
+# https://docs.fedoraproject.org/en-US/packaging-guidelines/Python/
+%define __python3 /usr/bin/python3.9
+
 Name:          skupper-router
 Version:       {{{ git_dir_version }}}
 Release:       2.0.0%{?dist}
@@ -44,7 +47,10 @@ VCS:           {{{ git_dir_vcs }}}
 BuildRequires: gcc
 BuildRequires: gcc-c++
 BuildRequires: cmake
-BuildRequires: python3-devel
+# https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/assembly_installing-and-using-python_configuring-basic-system-settings#proc_installing-python-3_assembly_installing-and-using-python
+# https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html-single/packaging_and_distributing_software/index#con_common-macros-for-python-3-rpms_assembly_packaging-python-3-rpms
+%{?fedora:BuildRequires: python3.9}
+%{?rhel:BuildRequires: python39-devel}
 BuildRequires: openssl-devel
 BuildRequires: qpid-proton-c-devel >= %{proton_minimum_version}
 BuildRequires: libwebsockets-devel >= %{libwebsockets_minimum_version}