sage_bootstrap.package.Package.tarball_pattern: Simplify code

Author: Matthias Koeppe

build/sage_bootstrap/package.py

@@ -148,7 +148,7 @@ def tarball_filename(self):
         """
         pattern = self.tarball_pattern
         if pattern:
-            return self.tarball_pattern.replace('VERSION', self.version)
+            return pattern.replace('VERSION', self.version)
         else:
             return None