spec: Use Python 3 for building when possible

While RHEL / CentOS are still using Python 2 for the time being,
Fedora has already switched to Python 3 as the default Python
interpreter a while ago, so on that OS it doesn't make sense to
drag in Python 2 anymore; the same applies to future RHEL versions.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

libvirt.spec.in

@@ -202,6 +202,13 @@
     %define with_bash_completion  0%{!?_without_bash_completion:1}
 %endif
 
+# Use Python 3 when possible, Python 2 otherwise
+%if 0%{?fedora} || 0%{?rhel} > 7
+    %define python python3
+%else
+    %define python python2
+%endif
+
 
 %if %{with_qemu} || %{with_lxc} || %{with_uml}
 # numad is used to manage the CPU and memory placement dynamically,
@@ -305,7 +312,7 @@ BuildRequires: perl-interpreter
 %else
 BuildRequires: perl
 %endif
-BuildRequires: python2
+BuildRequires: %{python}
 %if %{with_systemd}
 BuildRequires: systemd-units
 %endif