use %{__python3} macro

gutenprint.spec

@@ -4,7 +4,7 @@
 Name:           gutenprint
 Summary:        Printer Drivers Package
 Version:        5.2.14
-Release:        1%{?dist}
+Release:        2%{?dist}
 URL:            http://gimp-print.sourceforge.net/
 Source0:        http://downloads.sourceforge.net/gimp-print/%{name}-%{version}.tar.bz2
 # Post-install script to update CUPS native PPDs.
@@ -41,6 +41,9 @@ BuildRequires:  python3-cups
 # autoreconf
 BuildRequires: autoconf automake libtool
 
+# needed for defining %%{__python3} macro for prep phase
+BuildRequires: python3-devel
+
 ## NOTE ##
 # The README file in this package contains suggestions from upstream
 # on how to package this software. I'd be inclined to follow those
@@ -123,6 +126,9 @@ Epson, HP and compatible printers.
 
 cp %{SOURCE1} src/cups/cups-genppdupdate.in
 
+#shebang can change between releases - use %%{__python3} macro
+sed -i -e 's,^#!/usr/bin/python3,#!%{__python3},' src/cups/cups-genppdupdate.in
+
 # Python 3.6 invalid escape sequence deprecation fixes, COPYING as license (bug #1448303)
 %patch6 -p1 -b .python36syntax
 
@@ -240,6 +246,9 @@ exit 0
 %{_mandir}/man8/cups-genppd*.8*
 
 %changelog
+* Wed Jul 11 2018 Zdenek Dohnal <zdohnal@redhat.com> - 5.2.14-2
+- use %%{__python3} macro
+
 * Thu Jun 28 2018 Zdenek Dohnal <zdohnal@redhat.com> - 5.2.14-1
 - 5.2.14