mingw: Use Python 3 for building

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.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

mingw-libvirt.spec.in

@@ -72,7 +72,7 @@ BuildRequires:  pkgconfig
 # Need native version for msgfmt
 BuildRequires:  gettext
 BuildRequires:  libxslt
-BuildRequires:  python2
+BuildRequires:  python3
 %if 0%{?fedora} >= 27
 BuildRequires:  perl-interpreter
 %else