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/phases.py

@@ -463,11 +463,11 b' class phasecache:'
     def replace(self, phcache):
         """replace all values in 'self' with content of phcache"""
         for a in (
-            b'phaseroots',
-            b'dirty',
-            b'opener',
-            b'_loadedrevslen',
-            b'_phasesets',
+            'phaseroots',
+            'dirty',
+            'opener',
+            '_loadedrevslen',
+            '_phasesets',
         ):
             setattr(self, a, getattr(phcache, a))