Merge pull request swiftlang#32859 from compnerd/empirically-testing

validation: make test_shell Python3 friendly

Author: compnerd

utils/swift_build_support/swift_build_support/shell.py

@@ -137,7 +137,7 @@ def capture(command, stderr=None, env=None, dry_run=None, echo=True,
         return str(out.decode())
     except subprocess.CalledProcessError as e:
         if allow_non_zero_exit:
-            return e.output
+            return str(e.output.decode())
         if optional:
             return None
         _fatal_error(