some py3 fixes for gap and gap3

src/sage/interfaces/gap.py

@@ -514,8 +514,8 @@ def load_package(self, pkg, verbose=False):
             RuntimeError: Error loading Gap package chevie. You may want to install gap_packages SPKG.
         """
         if verbose:
-            print("Loading GAP package {}" % pkg)
-        x = self.eval('LoadPackage("%s")'%pkg)
+            print("Loading GAP package {}".format(pkg))
+        x = self.eval('LoadPackage("{}")'.format(pkg))
         if x == 'fail':
             raise RuntimeError("Error loading Gap package "+str(pkg)+". "+
                                "You may want to install gap_packages SPKG.")

src/sage/interfaces/gap3.py

@@ -231,6 +231,8 @@
 from sage.misc.cachefunc import cached_method
 from sage.interfaces.expect import Expect
 from sage.interfaces.gap import Gap_generic, GapElement_generic
+from sage.cpython.string import bytes_to_str
+
 
 # gap3_cmd should point to the gap3 executable
 gap3_cmd = 'gap3'
@@ -415,7 +417,8 @@ def _execute_line(self, line, wait_for_prompt=True, expect_eof=False):
         # detect it. So we test for a syntax error explicitly.
         normal_output, error_output = \
             super(Gap3, self)._execute_line(line, wait_for_prompt=True, expect_eof=False)
-        if normal_output.startswith("Syntax error:"):
+        normal = bytes_to_str(normal_output)
+        if normal.startswith("Syntax error:"):
             normal_output, error_output = "", normal_output
         return (normal_output, error_output)