attr: don't attempt to .encode() a str on Python 2

Otherwise it coerces automatically.

Differential Revision: https://phab.mercurial-scm.org/D6048

mercurial/thirdparty/attr/_make.py

@@ -555,7 +555,10 b' def _add_init(cls, frozen):'
 
     # We cache the generated init methods for the same kinds of attributes.
     sha1 = hashlib.sha1()
-    sha1.update(repr(attrs).encode("utf-8"))
+    r = repr(attrs)
+    if not isinstance(r, bytes):
+        r = r.encode('utf-8')
+    sha1.update(r)
     unique_filename = "<attrs generated init {0}>".format(
         sha1.hexdigest()
     )