attr: make some docstrings raw strings

This avoids a SyntaxWarning in Python 3.8 due to invalid
\ escapes.

Differential Revision: https://phab.mercurial-scm.org/D5817

mercurial/thirdparty/attr/_make.py

@@ -56,7 +56,7 b' Sentinel to indicate the lack of a value'
 def attr(default=NOTHING, validator=None,
          repr=True, cmp=True, hash=None, init=True,
          convert=None, metadata={}):
-    """
+    r"""
     Create a new attribute on a class.
 
     ..  warning::

mercurial/thirdparty/attr/filters.py

@@ -19,7 +19,7 b' def _split_what(what):'
 
 
 def include(*what):
-    """
+    r"""
     Whitelist *what*.
 
     :param what: What to whitelist.
@@ -36,7 +36,7 b' def include(*what):'
 
 
 def exclude(*what):
-    """
+    r"""
     Blacklist *what*.
 
     :param what: What to blacklist.