style: always use `x is not None` instead of `not x is None`

Differential Revision: https://phab.mercurial-scm.org/D842

mercurial/keepalive.py

@@ -393,7 +393,7 b' class HTTPResponse(httplib.HTTPResponse)'
     def read(self, amt=None):
         # the _rbuf test is only in this first if for speed.  It's not
         # logically necessary
-        if self._rbuf and not amt is None:
+        if self._rbuf and amt is not None:
             L = len(self._rbuf)
             if amt > L:
                 amt -= L

mercurial/pure/parsers.py

@@ -80,7 +80,7 b' class IndexObject(BaseIndexObject):'
         return i * indexsize
 
     def __delitem__(self, i):
-        if not isinstance(i, slice) or not i.stop == -1 or not i.step is None:
+        if not isinstance(i, slice) or not i.stop == -1 or i.step is not None:
             raise ValueError("deleting slices only supports a:-1 with step 1")
         i = self._fix_index(i.start)
         if i < self._lgt:
@@ -114,7 +114,7 b' class InlinedIndexObject(BaseIndexObject'
         return count
 
     def __delitem__(self, i):
-        if not isinstance(i, slice) or not i.stop == -1 or not i.step is None:
+        if not isinstance(i, slice) or not i.stop == -1 or i.step is not None:
             raise ValueError("deleting slices only supports a:-1 with step 1")
         i = self._fix_index(i.start)
         if i < self._lgt: