safehasattr: pass attribute name as string instead of bytes

This is a step toward replacing `util.safehasattr` usage with plain `hasattr`.
The builtin function behave poorly in Python2 but this was fixed in Python3.

These change are done one by one as they tend to have a small odd to trigger
puzzling breackage.

mercurial/templatefilters.py

@@ -339,7 +339,7 b' def json(obj, paranoid=True):'
         raise error.ProgrammingError(
             b'Mercurial only does output with bytes: %r' % obj
         )
-    elif util.safehasattr(obj, b'keys'):
+    elif util.safehasattr(obj, 'keys'):
         out = [
             b'"%s": %s'
             % (encoding.jsonescape(k, paranoid=paranoid), json(v, paranoid))